The electrostatic attractions and repulsions between bodies of

measurable dimensions are, of all the manifestations of this force, the

first so-called _electrical_ phenomena noted. But though they have been

known to us for many centuries, the precise nature of the mechanism

concerned in these actions is still unknown to us, and has not been even

quite satisfactorily explained. What kind of mechanism must that be? We

cannot help wondering when we observe two magnets attracting and

repelling each other with a force of hundreds of pounds with apparently

nothing between them. We have in our commercial dynamos magnets capable

of sustaining in mid-air tons of weight. But what are even these forces

acting between magnets when compared with the tremendous attractions and

repulsions produced by electrostatic force, to which there is apparently

no limit as to intensity. In lightning discharges bodies are often

charged to so high a potential that they are thrown away with

inconceivable force and torn asunder or shattered into fragments. Still

even such effects cannot compare with the attractions and repulsions

which exist between charged molecules or atoms, and which are sufficient

to project them with speeds of many kilometres a second, so that under

their violent impact bodies are rendered highly incandescent and are

volatilized. It is of special interest for the thinker who inquires into

the nature of these forces to note that whereas the actions between

individual molecules or atoms occur seemingly under any conditions, the

attractions and repulsions of bodies of measurable dimensions imply a

medium possessing insulating properties. So, if air, either by being

rarefied or heated, is rendered more or less conducting, these actions

between two electrified bodies practically cease, while the actions

between the individual atoms continue to manifest themselves.

An experiment may serve as an illustration and as a means of bringing

out other features of interest. Some time ago I showed that a lamp

filament or wire mounted in a bulb and connected to one of the terminals

of a high tension secondary coil is set spinning, the top of the

filament generally describing a circle. This vibration was very

energetic when the air in the bulb was at ordinary pressure and became

less energetic when the air in the bulb was strongly compressed. It

ceased altogether when the air was exhausted so as to become

comparatively good conducting. I found at that time that no vibration

took place when the bulb was very highly exhausted. But I conjectured

that the vibration which I ascribed to the electrostatic action between

the walls of the bulb and the filament should take place also in a

highly exhausted bulb. To test this under conditions which were more

favorable, a bulb like the one in Fig. 174, was constructed. It

comprised a globe _b_, in the neck of which was sealed a platinum wire

_w_ carrying a thin lamp filament _f_. In the lower part of the globe a

tube _t_ was sealed so as to surround the filament. The exhaustion was

carried as far as it was practicable with the apparatus employed.

This bulb verified my expectation, for the filament was set spinning

when the current was turned on, and became incandescent. It also showed

another interesting feature, bearing upon the preceding remarks, namely,

when the filament had been kept incandescent some time, the narrow tube

and the space inside were brought to an elevated temperature, and as the

gas in the tube then became conducting, the electrostatic attraction

between the glass and the filament became very weak or ceased, and the

filament came to rest. When it came to rest it would glow far more

intensely. This was probably due to its assuming the position in the

centre of the tube where the molecular bombardment was most intense, and

also partly to the fact that the individual impacts were more violent

and that no part of the supplied energy was converted into mechanical

movement. Since, in accordance with accepted views, in this experiment

the incandescence must be attributed to the impacts of the particles,

molecules or atoms in the heated space, these particles must therefore,

in order to explain such action, be assumed to behave as independent

carriers of electric charges immersed in an insulating medium; yet there

is no attractive force between the glass tube and the filament because

the space in the tube is, as a whole, conducting.

It is of some interest to observe in this connection that whereas the

attraction between two electrified bodies may cease owing to the

impairing of the insulating power of the medium in which they are

immersed, the repulsion between the bodies may still be observed. This

may be explained in a plausible way. When the bodies are placed at some

distance in a poorly conducting medium, such as slightly warmed or

rarefied air, and are suddenly electrified, opposite electric charges

being imparted to them, these charges equalize more or less by leakage

through the air. But if the bodies are similarly electrified, there is

less opportunity afforded for such dissipation, hence the repulsion

observed in such case is greater than the attraction. Repulsive actions

in a gaseous medium are however, as Prof. Crookes has shown, enhanced by

molecular bombardment.

ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA.

So far, I have considered principally effects produced by a varying

electrostatic force in an insulating medium, such as air. When such a

force is acting upon a conducting body of measurable dimensions, it

causes within the same, or on its surface, displacements of the

electricity and gives rise to electric currents, and these produce

another kind of phenomena, some of which I shall presently endeavor to

illustrate. In presenting this second class of electrical effects, I

will avail myself principally of such as are producible without any

return circuit, hoping to interest you the more by presenting these

phenomena in a more or less novel aspect.

It has been a long time customary, owing to the limited experience with

vibratory currents, to consider an electric current as something

circulating in a closed conducting path. It was astonishing at first to

realize that a current may flow through the conducting path even if the

latter be interrupted, and it was still more surprising to learn, that

sometimes it may be even easier to make a current flow under such

conditions than through a closed path. But that old idea is gradually

disappearing, even among practical men, and will soon be entirely

forgotten.

[Illustration: FIG. 175.]

If I connect an insulated metal plate P, Fig. 175, to one of the

terminals T of the induction coil by means of a wire, though this plate

be very well insulated, a current passes through the wire when the coil

is set to work. First I wish to give you evidence that there _is_ a

current passing through the connecting wire. An obvious way of

demonstrating this is to insert between the terminal of the coil and the

insulated plate a very thin platinum or german silver wire _w_ and bring

the latter to incandescence or fusion by the current. This requires a

rather large plate or else current impulses of very high potential and

frequency. Another way is to take a coil C, Fig. 175, containing many

turns of thin insulated wire and to insert the same in the path of the

current to the plate. When I connect one of the ends of the coil to the

wire leading to another insulated plate P_{1}, and its other end to the

terminal T_{1} of the induction coil, and set the latter to work, a

current passes through the inserted coil C and the existence of the

current may be made manifest in various ways. For instance, I insert an

iron core _i_ within the coil. The current being one of very high

frequency, will, if it be of some strength, soon bring the iron core to

a noticeably higher temperature, as the hysteresis and current losses

are great with such high frequencies. One might take a core of some

size, laminated or not, it would matter little; but ordinary iron wire

1/16th or 1/8th of an inch thick is suitable for the purpose. While the

induction coil is working, a current traverses the inserted coil and

only a few moments are sufficient to bring the iron wire _i_ to an

elevated temperature sufficient to soften the sealing-wax _s_, and cause

a paper washer _p_ fastened by it to the iron wire to fall off. But with

the apparatus such as I have here, other, much more interesting,

demonstrations of this kind can be made. I have a secondary S, Fig 176,

of coarse wire, wound upon a coil similar to the first. In the preceding

experiment the current through the coil C, Fig. 175, was very small, but

there being many turns a strong heating effect was, nevertheless,

produced in the iron wire. Had I passed that current through a conductor

in order to show the heating of the latter, the current might have been

too small to produce the effect desired. But with this coil provided

with a secondary winding, I can now transform the feeble current of high

tension which passes through the primary P into a strong secondary

current of low tension, and this current will quite certainly do what I

expect. In a small glass tube (_t_, Fig. 176), I have enclosed a coiled

platinum wire, _w_, this merely in order to protect the wire. On each

end of the glass tube is sealed a terminal of stout wire to which one of

the ends of the platinum wire _w_, is connected. I join the terminals of

the secondary coil to these terminals and insert the primary _p_,

between the insulated plate P_{1}, and the terminal T_{1}, of the

induction coil as before. The latter being set to work, instantly the

platinum wire _w_ is rendered incandescent and can be fused, even if it

be very thick.

[Illustration: FIG. 176.]

Instead of the platinum wire I now take an ordinary 50-volt 16 C. P.

lamp. When I set the induction coil in operation the lamp filament is

brought to high incandescence. It is, however, not necessary to use the

insulated plate, for the lamp (_l_, Fig. 177) is rendered incandescent

even if the plate P_{1} be disconnected. The secondary may also be

connected to the primary as indicated by the dotted line in Fig. 177, to

do away more or less with the electrostatic induction or to modify the

action otherwise.

[Illustration: FIG. 177.]

I may here call attention to a number of interesting observations with

the lamp. First, I disconnect one of the terminals of the lamp from the

secondary S. When the induction coil plays, a glow is noted which fills

the whole bulb. This glow is due to electrostatic induction. It

increases when the bulb is grasped with the hand, and the capacity of

the experimenter's body thus added to the secondary circuit. The

secondary, in effect, is equivalent to a metallic coating, which would

be placed near the primary. If the secondary, or its equivalent, the

coating, were placed symmetrically to the primary, the electrostatic

induction would be nil under ordinary conditions, that is, when a

primary return circuit is used, as both halves would neutralize each

other. The secondary _is_ in fact placed symmetrically to the primary,

but the action of both halves of the latter, when only one of its ends

is connected to the induction coil, is not exactly equal; hence

electrostatic induction takes place, and hence the glow in the bulb. I

can nearly equalize the action of both halves of the primary by

connecting the other, free end of the same to the insulated plate, as in

the preceding experiment. When the plate is connected, the glow

disappears. With a smaller plate it would not entirely disappear and

then it would contribute to the brightness of the filament when the

secondary is closed, by warming the air in the bulb.

[Illustration: FIG. 178a.]

[Illustration: FIG. 178b.]

[Illustration: FIG. 179a.]

[Illustration: FIG. 179b.]

To demonstrate another interesting feature, I have adjusted the coils

used in a certain way. I first connect both the terminals of the lamp to

the secondary, one end of the primary being connected to the terminal

T_{1} of the induction coil and the other to the insulated plate P_{1}

as before. When the current is turned on, the lamp glows brightly, as

shown in Fig. 178_b_, in which C is a fine wire coil and S a coarse wire

secondary wound upon it. If the insulated plate P_{1} is disconnected,

leaving one of the ends _a_ of the primary insulated, the filament

becomes dark or generally it diminishes in brightness (Fig. 178_a_).

Connecting again the plate P_{1} and raising the frequency of the

current, I make the filament quite dark or barely red (Fig. 179_b_).

Once more I will disconnect the plate. One will of course infer that

when the plate is disconnected, the current through the primary will be

weakened, that therefore the E. M. F. will fall in the secondary S, and

that the brightness of the lamp will diminish. This might be the case

and the result can be secured by an easy adjustment of the coils; also

by varying the frequency and potential of the currents. But it is

perhaps of greater interest to note, that the lamp increases in

brightness when the plate is disconnected (Fig. 179_a_). In this case

all the energy the primary receives is now sunk into it, like the charge

of a battery in an ocean cable, but most of that energy is recovered

through the secondary and used to light the lamp. The current traversing

the primary is strongest at the end _b_ which is connected to the

terminal T_{1} of the induction coil, and diminishes in strength towards

the remote end _a_. But the dynamic inductive effect exerted upon the

secondary S is now greater than before, when the suspended plate was

connected to the primary. These results might have been produced by a

number of causes. For instance, the plate P_{1} being connected, the

reaction from the coil C may be such as to diminish the potential at the

terminal T_{1} of the induction coil, and therefore weaken the current

through the primary of the coil C. Or the disconnecting of the plate

may diminish the capacity effect with relation to the primary of the

latter coil to such an extent that the current through it is diminished,

though the potential at the terminal T_{1} of the induction coil may be

the same or even higher. Or the result might have been produced by the

change of phase of the primary and secondary currents and consequent

reaction. But the chief determining factor is the relation of the

self-induction and capacity of coil C and plate P_{1} and the frequency

of the currents. The greater brightness of the filament in Fig. 179_a_,

is, however, in part due to the heating of the rarefied gas in the lamp

by electrostatic induction, which, as before remarked, is greater when

the suspended plate is disconnected.

Still another feature of some interest I may here bring to your

attention. When the insulated plate is disconnected and the secondary of

the coil opened, by approaching a small object to the secondary, but

very small sparks can be drawn from it, showing that the electrostatic

induction is small in this case. But upon the secondary being closed

upon itself or through the lamp, the filament glowing brightly, strong

sparks are obtained from the secondary. The electrostatic induction is

now much greater, because the closed secondary determines a greater flow

of current through the primary and principally through that half of it

which is connected to the induction coil. If now the bulb be grasped

with the hand, the capacity of the secondary with reference to the

primary is augmented by the experimenter's body and the luminosity of

the filament is increased, the incandescence now being due partly to the

flow of current through the filament and partly to the molecular

bombardment of the rarefied gas in the bulb.

The preceding experiments will have prepared one for the next following

results of interest, obtained in the course of these investigations.

Since I can pass a current through an insulated wire merely by

connecting one of its ends to the source of electrical energy, since I

can induce by it another current, magnetize an iron core, and, in short,

perform all operations as though a return circuit were used, clearly I

can also drive a motor by the aid of only one wire. On a former occasion

I have described a simple form of motor comprising a single exciting

coil, an iron core and disc. Fig. 180 illustrates a modified way of

operating such an alternate current motor by currents induced in a

transformer connected to one lead, and several other arrangements of

circuits for operating a certain class of alternating motors founded on

the action of currents of differing phase. In view of the present state

of the art it is thought sufficient to describe these arrangements in a

few words only. The diagram, Fig. 180 II., shows a primary coil P,

connected with one of its ends to the line L leading from a high tension

transformer terminal T_{1}. In inductive relation to this primary P is a

secondary S of coarse wire in the circuit of which is a coil _c_. The

currents induced in the secondary energize the iron core _i_, which is

preferably, but not necessarily, subdivided, and set the metal disc _d_

in rotation. Such a motor M_{2} as diagramatically shown in Fig. 180

II., has been called a "magnetic lag motor," but this expression may be

objected to by those who attribute the rotation of the disc to eddy

currents circulating in minute paths when the core _i_ is finally

subdivided. In order to operate such a motor effectively on the plan

indicated, the frequencies should not be too high, not more than four or

five thousand, though the rotation is produced even with ten thousand

per second, or more.

In Fig. 180 I., a motor M_{1} having two energizing circuits, A and B,

is diagrammatically indicated. The circuit A is connected to the line L

and in series with it is a primary P, which may have its free end

connected to an insulated plate P_{1}, such connection being indicated

by the dotted lines. The other motor circuit B is connected to the

secondary S which is in inductive relation to the primary P. When the

transformer terminal T_{1} is alternately electrified, currents traverse

the open line L and also circuit A and primary P. The currents through

the latter induce secondary currents in the circuit S, which pass

through the energizing coil B of the motor. The currents through the

secondary S and those through the primary P differ in phase 90 degrees,

or nearly so, and are capable of rotating an armature placed in

inductive relation to the circuits A and B.

In Fig. 180 III., a similar motor M_{3} with two energizing circuits

A_{1} and B_{1} is illustrated. A primary P, connected with one of its

ends to the line L has a secondary S, which is preferably wound for a

tolerably high E. M. F., and to which the two energizing circuits of the

motor are connected, one directly to the ends of the secondary and the

other through a condenser C, by the action of which the currents

traversing the circuit A_{1} and B_{1} are made to differ in phase.

[Illustration: FIG. 180.]

[Illustration: FIG. 181.]

[Illustration: FIG. 182.]

In Fig. 180 IV., still another arrangement is shown. In this case two

primaries P_{1} and P_{2} are connected to the line L, one through a

condenser C of small capacity, and the other directly. The primaries are

provided with secondaries S_{1} and S_{2} which are in series with the

energizing circuits, A_{2} and B_{2} and a motor M_{3}, the condenser C

again serving to produce the requisite difference in the phase of the

currents traversing the motor circuits. As such phase motors with two or

more circuits are now well known in the art, they have been here

illustrated diagrammatically. No difficulty whatever is found in

operating a motor in the manner indicated, or in similar ways; and

although such experiments up to this day present only scientific

interest, they may at a period not far distant, be carried out with

practical objects in view.

It is thought useful to devote here a few remarks to the subject of

operating devices of all kinds by means of only one leading wire. It is

quite obvious, that when high-frequency currents are made use of, ground

connections are--at least when the E. M. F. of the currents is

great--better than a return wire. Such ground connections are

objectionable with steady or low frequency currents on account of

destructive chemical actions of the former and disturbing influences

exerted by both on the neighboring circuits; but with high frequencies

these actions practically do not exist. Still, even ground connections

become superfluous when the E. M. F. is very high, for soon a condition

is reached, when the current may be passed more economically through

open, than through closed, conductors. Remote as might seem an

industrial application of such single wire transmission of energy to one

not experienced in such lines of experiment, it will not seem so to

anyone who for some time has carried on investigations of such nature.

Indeed I cannot see why such a plan should not be practicable. Nor

should it be thought that for carrying out such a plan currents of very

high frequency are expressly required, for just as soon as potentials of

say 30,000 volts are used, the single wire transmission may be effected

with low frequencies, and experiments have been made by me from which

these inferences are made.

When the frequencies are very high it has been found in laboratory

practice quite easy to regulate the effects in the manner shown in

diagram Fig. 181. Here two primaries P and P_{1} are shown, each

connected with one of its ends to the line L and with the other end to

the condenser plates C and C, respectively. Near these are placed other

condenser plates C_{1} and C_{1}, the former being connected to the line

L and the latter to an insulated larger plate P_{2}. On the primaries

are wound secondaries S and S_{1}, of coarse wire, connected to the

devices _d_ and _l_ respectively. By varying the distances of the

condenser plates C and C_{1}, and C and C_{1} the currents through the

secondaries S and S_{1} are varied in intensity. The curious feature is

the great sensitiveness, the slightest change in the distance of the

plates producing considerable variations in the intensity or strength of

the currents. The sensitiveness may be rendered extreme by making the

frequency such, that the primary itself, without any plate attached to

its free end, satisfies, in conjunction with the closed secondary, the

condition of resonance. In such condition an extremely small change in

the capacity of the free terminal produces great variations. For

instance, I have been able to adjust the conditions so that the mere

approach of a person to the coil produces a considerable change in the

brightness of the lamps attached to the secondary. Such observations and

experiments possess, of course, at present, chiefly scientific interest,

but they may soon become of practical importance.

Very high frequencies are of course not practicable with motors on

account of the necessity of employing iron cores. But one may use sudden

discharges of low frequency and thus obtain certain advantages of

high-frequency currents without rendering the iron core entirely

incapable of following the changes and without entailing a very great

expenditure of energy in the core. I have found it quite practicable to

operate with such low frequency disruptive discharges of condensers,

alternating-current motors. A certain class of such motors which I

advanced a few years ago, which contain closed secondary circuits, will

rotate quite vigorously when the discharges are directed through the

exciting coils. One reason that such a motor operates so well with these

discharges is that the difference of phase between the primary and

secondary currents is 90 degrees, which is generally not the case with

harmonically rising and falling currents of low frequency. It might not

be without interest to show an experiment with a simple motor of this

kind, inasmuch as it is commonly thought that disruptive discharges are

unsuitable for such purposes. The motor is illustrated in Fig. 182. It

comprises a rather large iron core _i_ with slots on the top into which

are embedded thick copper washers _c c_. In proximity to the core is a

freely-movable metal disc D. The core is provided with a primary

exciting coil C_{1} the ends _a_ and _b_ of which are connected to the

terminals of the secondary S of an ordinary transformer, the primary P

of the latter being connected to an alternating distribution circuit or

generator G of low or moderate frequency. The terminals of the secondary

S are attached to a condenser C which discharges through an air gap _d

d_ which may be placed in series or shunt to the coil C_{1}. When the

conditions are properly chosen the disc D rotates with considerable

effort and the iron core _i_ does not get very perceptibly hot. With

currents from a high-frequency alternator, on the contrary, the core

gets rapidly hot and the disc rotates with a much smaller effort. To

perform the experiment properly it should be first ascertained that the

disc D is not set in rotation when the discharge is not occurring at _d

d_. It is preferable to use a large iron core and a condenser of large

capacity so as to bring the superimposed quicker oscillation to a very

low pitch or to do away with it entirely. By observing certain

elementary rules I have also found it practicable to operate ordinary

series or shunt direct-current motors with such disruptive discharges,

and this can be done with or without a return wire.

IMPEDANCE PHENOMENA.

Among the various current phenomena observed, perhaps the most

interesting are those of impedance presented by conductors to currents

varying at a rapid rate. In my first paper before the American Institute

of Electrical Engineers, I have described a few striking observations of

this kind. Thus I showed that when such currents or sudden discharges

are passed through a thick metal bar there may be points on the bar only

a few inches apart, which have a sufficient potential difference between

them to maintain at bright incandescence an ordinary filament lamp. I

have also described the curious behavior of rarefied gas surrounding a

conductor, due to such sudden rushes of current. These phenomena have

since been more carefully studied and one or two novel experiments of

this kind are deemed of sufficient interest to be described here.

Referring to Fig. 183_a_, B and B_{1} are very stout copper bars

connected at their lower ends to plates C and C_{1}, respectively, of a

condenser, the opposite plates of the latter being connected to the

terminals of the secondary S of a high-tension transformer, the primary

P of which is supplied with alternating currents from an ordinary

low-frequency dynamo G or distribution circuit. The condenser

discharges through an adjustable gap _d d_ as usual. By establishing a

rapid vibration it was found quite easy to perform the following curious

experiment. The bars B and B_{1} were joined at the top by a low-voltage

lamp l_{3}; a little lower was placed by means of clamps _c c_, a

50-volt lamp l_{2}; and still lower another 100-volt lamp l_{1}; and

finally, at a certain distance below the latter lamp, an exhausted tube

T. By carefully determining the positions of these devices it was found

practicable to maintain them all at their proper illuminating power. Yet

they were all connected in multiple arc to the two stout copper bars and

required widely different pressures. This experiment requires of course

some time for adjustment but is quite easily performed.

[Illustration: FIGS. 183a, 183b and 183c.]

In Figs. 183_b_ and 183_c_, two other experiments are illustrated which,

unlike the previous experiment, do not require very careful adjustments.

In Fig. 183_b_, two lamps, l_{1} and l_{2}, the former a 100-volt

and the latter a 50-volt are placed in certain positions as indicated,

the 100-volt lamp being below the 50-volt lamp. When the arc is playing

at _d d_ and the sudden discharges are passed through the bars B B_{1},

the 50-volt lamp will, as a rule, burn brightly, or at least this result

is easily secured, while the 100-volt lamp will burn very low or remain

quite dark, Fig. 183_b_. Now the bars B B_{1} may be joined at the top

by a thick cross bar B_{2} and it is quite easy to maintain the 100-volt

lamp at full candle-power while the 50-volt lamp remains dark, Fig.

183_c_. These results, as I have pointed out previously, should not be

considered to be due exactly to frequency but rather to the time rate of

change which may be great, even with low frequencies. A great many other

results of the same kind, equally interesting, especially to those who

are only used to manipulate steady currents, may be obtained and they

afford precious clues in investigating the nature of electric currents.

In the preceding experiments I have already had occasion to show some

light phenomena and it would now be proper to study these in particular;

but to make this investigation more complete I think it necessary to

make first a few remarks on the subject of electrical resonance which

has to be always observed in carrying out these experiments.

ON ELECTRICAL RESONANCE.

The effects of resonance are being more and more noted by engineers and

are becoming of great importance in the practical operation of apparatus

of all kinds with alternating currents. A few general remarks may

therefore be made concerning these effects. It is clear, that if we

succeed in employing the effects of resonance practically in the

operation of electric devices the return wire will, as a matter of

course, become unnecessary, for the electric vibration may be conveyed

with one wire just as well as, and sometimes even better than, with two.

The question first to answer is, then, whether pure resonance effects

are producible. Theory and experiment both show that such is impossible

in Nature, for as the oscillation becomes more and more vigorous, the

losses in the vibrating bodies and environing media rapidly increase and

necessarily check the vibration which otherwise would go on increasing

forever. It is a fortunate circumstance that pure resonance is not

producible, for if it were there is no telling what dangers might not

lie in wait for the innocent experimenter. But to a certain degree

resonance is producible, the magnitude of the effects being limited by

the imperfect conductivity and imperfect elasticity of the media or,

generally stated, by frictional losses. The smaller these losses, the

more striking are the effects. The same is the case in mechanical

vibration. A stout steel bar may be set in vibration by drops of water

falling upon it at proper intervals; and with glass, which is more

perfectly elastic, the resonance effect is still more remarkable, for a

goblet may be burst by singing into it a note of the proper pitch. The

electrical resonance is the more perfectly attained, the smaller the

resistance or the impedance of the conducting path and the more perfect

the dielectric. In a Leyden jar discharging through a short stranded

cable of thin wires these requirements are probably best fulfilled, and

the resonance effects are therefore very prominent. Such is not the case

with dynamo machines, transformers and their circuits, or with

commercial apparatus in general in which the presence of iron cores

complicates the action or renders it impossible. In regard to Leyden

jars with which resonance effects are frequently demonstrated, I would

say that the effects observed are often _attributed_ but are seldom

_due_ to true resonance, for an error is quite easily made in this

respect. This may be undoubtedly demonstrated by the following

experiment. Take, for instance, two large insulated metallic plates or

spheres which I shall designate A and B; place them at a certain small

distance apart and charge them from a frictional or influence machine to

a potential so high that just a slight increase of the difference of

potential between them will cause the small air or insulating space to

break down. This is easily reached by making a few preliminary trials.

If now another plate--fastened on an insulating handle and connected by

a wire to one of the terminals of a high tension secondary of an

induction coil, which is maintained in action by an alternator

(preferably high frequency)--is approached to one of the charged bodies

A or B, so as to be nearer to either one of them, the discharge will

invariably occur between them; at least it will, if the potential of the

coil in connection with the plate is sufficiently high. But the

explanation of this will soon be found in the fact that the approached

plate acts inductively upon the bodies A and B and causes a spark to

pass between them. When this spark occurs, the charges which were

previously imparted to these bodies from the influence machine, must

needs be lost, since the bodies are brought in electrical connection

through the arc formed. Now this arc is formed whether there be

resonance or not. But even if the spark would not be produced, still

there is an alternating E. M. F. set up between the bodies when the

plate is brought near one of them; therefore the approach of the plate,

if it _does_ not always actually, will, at any rate, _tend_ to break

down the air space by inductive action. Instead of the spheres or plates

A and B we may take the coatings of a Leyden jar with the same result,

and in place of the machine,--which is a high frequency alternator

preferably, because it is more suitable for the experiment and also for

the argument,--we may take another Leyden jar or battery of jars. When

such jars are discharging through a circuit of low resistance the same

is traversed by currents of very high frequency. The plate may now be

connected to one of the coatings of the second jar, and when it is

brought near to the first jar just previously charged to a high

potential from an influence machine, the result is the same as before,

and the first jar will discharge through a small air space upon the

second being caused to discharge. But both jars and their circuits need

not be tuned any closer than a basso profundo is to the note produced by

a mosquito, as small sparks will be produced through the air space, or

at least the latter will be considerably more strained owing to the

setting up of an alternating E. M. F. by induction, which takes place

when one of the jars begins to discharge. Again another error of a

similar nature is quite easily made. If the circuits of the two jars are

run parallel and close together, and the experiment has been performed

of discharging one by the other, and now a coil of wire be added to one

of the circuits whereupon the experiment does not succeed, the

conclusion that this is due to the fact that the circuits are now not

tuned, would be far from being safe. For the two circuits act as

condenser coatings and the addition of the coil to one of them is

equivalent to bridging them, at the point where the coil is placed, by a

small condenser, and the effect of the latter might be to prevent the

spark from jumping through the discharge space by diminishing the

alternating E. M. F. acting across the same. All these remarks, and many

more which might be added but for fear of wandering too far from the

subject, are made with the pardonable intention of cautioning the

unsuspecting student, who might gain an entirely unwarranted opinion of

his skill at seeing every experiment succeed; but they are in no way

thrust upon the experienced as novel observations.

In order to make reliable observations of electric resonance effects it

is very desirable, if not necessary, to employ an alternator giving

currents which rise and fall harmonically, as in working with make and

break currents the observations are not always trustworthy, since many

phenomena, which depend on the rate of change, may be produced with

widely different frequencies. Even when making such observations with an

alternator one is apt to be mistaken. When a circuit is connected to an

alternator there are an indefinite number of values for capacity and

self-induction which, in conjunction, will satisfy the condition of

resonance. So there are in mechanics an infinite number of tuning forks

which will respond to a note of a certain pitch, or loaded springs which

have a definite period of vibration. But the resonance will be most

perfectly attained in that case in which the motion is effected with the

greatest freedom. Now in mechanics, considering the vibration in the

common medium--that is, air--it is of comparatively little importance

whether one tuning fork be somewhat larger than another, because the

losses in the air are not very considerable. One may, of course, enclose

a tuning fork in an exhausted vessel and by thus reducing the air

resistance to a minimum obtain better resonant action. Still the

difference would not be very great. But it would make a great difference

if the tuning fork were immersed in mercury. In the electrical vibration

it is of enormous importance to arrange the conditions so that the

vibration is effected with the greatest freedom. The magnitude of the

resonance effect depends, under otherwise equal conditions, on the

quantity of electricity set in motion or on the strength of the current

driven through the circuit. But the circuit opposes the passage of the

currents by reason of its impedance and therefore, to secure the best

action it is necessary to reduce the impedance to a minimum. It is

impossible to overcome it entirely, but merely in part, for the ohmic

resistance cannot be overcome. But when the frequency of the impulses is

very great, the flow of the current is practically determined by

self-induction. Now self-induction can be overcome by combining it with

capacity. If the relation between these is such, that at the frequency

used they annul each other, that is, have such values as to satisfy the

condition of resonance, and the greatest quantity of electricity is made

to flow through the external circuit, then the best result is obtained.

It is simpler and safer to join the condenser in series with the

self-induction. It is clear that in such combinations there will be,

for a given frequency, and considering only the fundamental vibration,

values which will give the best result, with the condenser in shunt to

the self-induction coil; of course more such values than with the

condenser in series. But practical conditions determine the selection.

In the latter case in performing the experiments one may take a small

self-induction and a large capacity or a small capacity and a large

self-induction, but the latter is preferable, because it is inconvenient

to adjust a large capacity by small steps. By taking a coil with a very

large self-induction the critical capacity is reduced to a very small

value, and the capacity of the coil itself may be sufficient. It is

easy, especially by observing certain artifices, to wind a coil through

which the impedance will be reduced to the value of the ohmic resistance

only; and for any coil there is, of course, a frequency at which the

maximum current will be made to pass through the coil. The observation

of the relation between self-induction, capacity and frequency is

becoming important in the operation of alternate current apparatus, such

as transformers or motors, because by a judicious determination of the

elements the employment of an expensive condenser becomes unnecessary.

Thus it is possible to pass through the coils of an alternating current

motor under the normal working conditions the required current with a

low E. M. F. and do away entirely with the false current, and the larger

the motor, the easier such a plan becomes practicable; but it is

necessary for this to employ currents of very high potential or high

frequency.

[Illustration: FIG. 184.]

In Fig. 184 I. is shown a plan which has been followed in the study of

the resonance effects by means of a high frequency alternator. C_{1} is

a coil of many turns, which is divided into small separate sections for

the purpose of adjustment. The final adjustment was made sometimes with

a few thin iron wires (though this is not always advisable) or with a

closed secondary. The coil C_{1} is connected with one of its ends to

the line L from the alternator G and with the other end to one of the

plates _c_ of a condenser c c_{1}, the plate (c_{1}) of the latter

being connected to a much larger plate P_{1}. In this manner both

capacity and self-induction were adjusted to suit the dynamo frequency.

As regards the rise of potential through resonant action, of course,

theoretically, it may amount to anything since it depends on

self-induction and resistance and since these may have any value. But in

practice one is limited in the selection of these values and besides

these, there are other limiting causes. One may start with, say, 1,000

volts and raise the E. M. F. to 50 times that value, but one cannot

start with 100,000 and raise it to ten times that value because of the

losses in the media which are great, especially if the frequency is

high. It should be possible to start with, for instance, two volts from

a high or low frequency circuit of a dynamo and raise the E. M. F. to

many hundred times that value. Thus coils of the proper dimensions might

be connected each with only one of its ends to the mains from a machine

of low E. M. F., and though the circuit of the machine would not be

closed in the ordinary acceptance of the term, yet the machine might be

burned out if a proper resonance effect would be obtained. I have not

been able to produce, nor have I observed with currents from a dynamo

machine, such great rises of potential. It is possible, if not probable,

that with currents obtained from apparatus containing iron the

disturbing influence of the latter is the cause that these theoretical

possibilities cannot be realized. But if such is the case I attribute it

solely to the hysteresis and Foucault current losses in the core.

Generally it was necessary to transform upward, when the E. M. F. was

very low, and usually an ordinary form of induction coil was employed,

but sometimes the arrangement illustrated in Fig. 184 II., has been

found to be convenient. In this case a coil C is made in a great many

sections, a few of these being used as a primary. In this manner both

primary and secondary are adjustable. One end of the coil is connected

to the line L_{1} from the alternator, and the other line L is connected

to the intermediate point of the coil. Such a coil with adjustable

primary and secondary will be found also convenient in experiments with

the disruptive discharge. When true resonance is obtained the top of the

wave must of course be on the free end of the coil as, for instance, at

the terminal of the phosphorescence bulb B. This is easily recognized

by observing the potential of a point on the wire _w_ near to the coil.

In connection with resonance effects and the problem of transmission of

energy over a single conductor which was previously considered, I would

say a few words on a subject which constantly fills my thoughts and

which concerns the welfare of all. I mean the transmission of

intelligible signals or perhaps even power to any distance without the

use of wires. I am becoming daily more convinced of the practicability

of the scheme; and though I know full well that the great majority of

scientific men will not believe that such results can be practically and

immediately realized, yet I think that all consider the developments in

recent years by a number of workers to have been such as to encourage

thought and experiment in this direction. My conviction has grown so

strong, that I no longer look upon this plan of energy or intelligence

transmission as a mere theoretical possibility, but as a serious problem

in electrical engineering, which must be carried out some day. The idea

of transmitting intelligence without wires is the natural outcome of the

most recent results of electrical investigations. Some enthusiasts have

expressed their belief that telephony to any distance by induction

through the air is possible. I cannot stretch my imagination so far, but

I do firmly believe that it is practicable to disturb by means of

powerful machines the electrostatic condition of the earth and thus

transmit intelligible signals and perhaps power. In fact, what is there

against the carrying out of such a scheme? We now know that electric

vibration may be transmitted through a single conductor. Why then not

try to avail ourselves of the earth for this purpose? We need not be

frightened by the idea of distance. To the weary wanderer counting the

mile-posts the earth may appear very large, but to that happiest of all

men, the astronomer, who gazes at the heavens and by their standard

judges the magnitude of our globe, it appears very small. And so I think

it must seem to the electrician, for when he considers the speed with

which an electric disturbance is propagated through the earth all his

ideas of distance must completely vanish.

A point of great importance would be first to know what is the capacity

of the earth? and what charge does it contain if electrified? Though we

have no positive evidence of a charged body existing in space without

other oppositely electrified bodies being near, there is a fair

probability that the earth is such a body, for by whatever process it

was separated from other bodies--and this is the accepted view of its

origin--it must have retained a charge, as occurs in all processes of

mechanical separation. If it be a charged body insulated in space its

capacity should be extremely small, less than one-thousandth of a farad.

But the upper strata of the air are conducting, and so, perhaps, is the

medium in free space beyond the atmosphere, and these may contain an

opposite charge. Then the capacity might be incomparably greater. In any

case it is of the greatest importance to get an idea of what quantity of

electricity the earth contains. It is difficult to say whether we shall

ever acquire this necessary knowledge, but there is hope that we may,

and that is, by means of electrical resonance. If ever we can ascertain

at what period the earth's charge, when disturbed, oscillates with

respect to an oppositely electrified system or known circuit, we shall

know a fact possibly of the greatest importance to the welfare of the

human race. I propose to seek for the period by means of an electrical

oscillator, or a source of alternating electric currents. One of the

terminals of the source would be connected to earth as, for instance, to

the city water mains, the other to an insulated body of large surface.

It is possible that the outer conducting air strata, or free space,

contain an opposite charge and that, together with the earth, they form

a condenser of very large capacity. In such case the period of vibration

may be very low and an alternating dynamo machine might serve for the

purpose of the experiment. I would then transform the current to a

potential as high as it would be found possible and connect the ends of

the high tension secondary to the ground and to the insulated body. By

varying the frequency of the currents and carefully observing the

potential of the insulated body and watching for the disturbance at

various neighboring points of the earth's surface resonance might be

detected. Should, as the majority of scientific men in all probability

believe, the period be extremely small, then a dynamo machine would not

do and a proper electrical oscillator would have to be produced and

perhaps it might not be possible to obtain such rapid vibrations. But

whether this be possible or not, and whether the earth contains a charge

or not, and whatever may be its period of vibration, it certainly is

possible--for of this we have daily evidence--to produce some electrical

disturbance sufficiently powerful to be perceptible by suitable

instruments at any point of the earth's surface.

[Illustration: FIG. 185.]

Assume that a source of alternating current S be connected, as in Fig.

185, with one of its terminals to earth (conveniently to the water

mains) and with the other to a body of large surface P. When the

electric oscillation is set up there will be a movement of electricity

in and out of P, and alternating currents will pass through the earth,

converging to, or diverging from, the point C where the ground

connection is made. In this manner neighboring points on the earth's

surface within a certain radius will be disturbed. But the disturbance

will diminish with the distance, and the distance at which the effect

will still be perceptible will depend on the quantity of electricity set

in motion. Since the body P is insulated, in order to displace a

considerable quantity, the potential of the source must be excessive,

since there would be limitations as to the surface of P. The conditions

might be adjusted so that the generator or source S will set up the same

electrical movement as though its circuit were closed. Thus it is

certainly practicable to impress an electric vibration at least of a

certain low period upon the earth by means of proper machinery. At what

distance such a vibration might be made perceptible can only be

conjectured. I have on another occasion considered the question how the

earth might behave to electric disturbances. There is no doubt that,

since in such an experiment the electrical density at the surface could

be but extremely small considering the size of the earth, the air would

not act as a very disturbing factor, and there would be not much energy

lost through the action of the air, which would be the case if the

density were great. Theoretically, then, it could not require a great

amount of energy to produce a disturbance perceptible at great distance,

or even all over the surface of the globe. Now, it is quite certain that

at any point within a certain radius of the source S a properly adjusted

self-induction and capacity device can be set in action by resonance.

But not only can this be done, but another source S_{1}, Fig. 185,

similar to S, or any number of such sources, can be set to work in

synchronism with the latter, and the vibration thus intensified and

spread over a large area, or a flow of electricity produced to or from

the source S_{1} if the same be of opposite phase to the source S. I

think that beyond doubt it is possible to operate electrical devices in

a city through the ground or pipe system by resonance from an electrical

oscillator located at a central point. But the practical solution of

this problem would be of incomparably smaller benefit to man than the

realization of the scheme of transmitting intelligence, or perhaps

power, to any distance through the earth or environing medium. If this

is at all possible, distance does not mean anything. Proper apparatus

must first be produced by means of which the problem can be attacked and

I have devoted much thought to this subject. I am firmly convinced that

it can be done and hope that we shall live to see it done.

ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CURRENTS OF HIGH

POTENTIAL AND GENERAL REMARKS RELATING TO THE SUBJECT.

Returning now to the light effects which it has been the chief object to

investigate, it is thought proper to divide these effects into four

classes: 1. Incandescence of a solid. 2. Phosphorescence. 3.

Incandescence or phosphorescence of a rarefied gas; and 4. Luminosity

produced in a gas at ordinary pressure. The first question is: How are

these luminous effects produced? In order to answer this question as

satisfactorily as I am able to do in the light of accepted views and

with the experience acquired, and to add some interest to this

demonstration, I shall dwell here upon a feature which I consider of

great importance, inasmuch as it promises, besides, to throw a better

light upon the nature of most of the phenomena produced by

high-frequency electric currents. I have on other occasions pointed out

the great importance of the presence of the rarefied gas, or atomic

medium in general, around the conductor through which alternate currents

of high frequency are passed, as regards the heating of the conductor by

the currents. My experiments, described some time ago, have shown that,

the higher the frequency and potential difference of the currents, the

more important becomes the rarefied gas in which the conductor is

immersed, as a factor of the heating. The potential difference, however,

is, as I then pointed out, a more important element than the frequency.

When both of these are sufficiently high, the heating may be almost

entirely due to the presence of the rarefied gas. The experiments to

follow will show the importance of the rarefied gas, or, generally, of

gas at ordinary or other pressure as regards the incandescence or other

luminous effects produced by currents of this kind.

I take two ordinary 50-volt 16 C. P. lamps which are in every respect

alike, with the exception, that one has been opened at the top and the

air has filled the bulb, while the other is at the ordinary degree of

exhaustion of commercial lamps. When I attach the lamp which is

exhausted to the terminal of the secondary of the coil, which I have

already used, as in experiments illustrated in Fig. 179_a_ for instance,

and turn on the current, the filament, as you have before seen, comes to

high incandescence. When I attach the second lamp, which is filled with

air, instead of the former, the filament still glows, but much less

brightly. This experiment illustrates only in part the truth of the

statements before made. The importance of the filament's being immersed

in rarefied gas is plainly noticeable but not to such a degree as might

be desirable. The reason is that the secondary of this coil is wound for

low tension, having only 150 turns, and the potential difference at the

terminals of the lamp is therefore small. Were I to take another coil

with many more turns in the secondary, the effect would be increased,

since it depends partially on the potential difference, as before

remarked. But since the effect likewise depends on the frequency, it

maybe properly stated that it depends on the time rate of the variation

of the potential difference. The greater this variation, the more

important becomes the gas as an element of heating. I can produce a much

greater rate of variation in another way, which, besides, has the

advantage of doing away with the objections, which might be made in the

experiment just shown, even if both the lamps were connected in series

or multiple arc to the coil, namely, that in consequence of the

reactions existing between the primary and secondary coil the

conclusions are rendered uncertain. This result I secure by charging,

from an ordinary transformer which is fed from the alternating current

supply station, a battery of condensers, and discharging the latter

directly through a circuit of small self-induction, as before

illustrated in Figs. 183_a_, 183_b_, and 183_c_.

[Illustration: FIG. 186a.]

[Illustration: FIG. 186b.]

[Illustration: FIG. 186c.]

In Figs. 186_a_, 186_b_ and 186_c_, the heavy copper bars B B_{1}, are

connected to the opposite coatings of a battery of condensers, or

generally in such way, that the high frequency or sudden discharges are

made to traverse them. I connect first an ordinary 50-volt incandescent

lamp to the bars by means of the clamps _c c_. The discharges being

passed through the lamp, the filament is rendered incandescent, though

the current through it is very small, and would not be nearly sufficient

to produce a visible effect under the conditions of ordinary use of the

lamp. Instead of this I now attach to the bars another lamp exactly like

the first, but with the seal broken off, the bulb being therefore filled

with air at ordinary pressure. When the discharges are directed through

the filament, as before, it does not become incandescent. But the result

might still be attributed to one of the many possible reactions. I

therefore connect both the lamps in multiple arc as illustrated in Fig.

186_a_. Passing the discharges through both the lamps, again the

filament in the exhausted lamp _l_ glows very brightly while that in the

non-exhausted lamp l_{1} remains dark, as previously. But it should

not be thought that the latter lamp is taking only a small fraction of

the energy supplied to both the lamps; on the contrary, it may consume a

considerable portion of the energy and it may become even hotter than

the one which burns brightly. In this experiment the potential

difference at the terminals of the lamps varies in sign theoretically

three to four million times a second. The ends of the filaments are

correspondingly electrified, and the gas in the bulbs is violently

agitated and a large portion of the supplied energy is thus converted

into heat. In the non-exhausted bulb, there being a few million times

more gas molecules than in the exhausted one, the bombardment, which is

most violent at the ends of the filament, in the neck of the bulb,

consumes a large portion of the energy without producing any visible

effect. The reason is that, there being many molecules, the bombardment

is quantitatively considerable, but the individual impacts are not very

violent, as the speeds of the molecules are comparatively small owing to

the small free path. In the exhausted bulb, on the contrary, the speeds

are very great, and the individual impacts are violent and therefore

better adapted to produce a visible effect. Besides, the convection of

heat is greater in the former bulb. In both the bulbs the current

traversing the filaments is very small, incomparably smaller than that

which they require on an ordinary low-frequency circuit. The potential

difference, however, at the ends of the filaments is very great and

might be possibly 20,000 volts or more, if the filaments were straight

and their ends far apart. In the ordinary lamp a spark generally occurs

between the ends of the filament or between the platinum wires outside,

before such a difference of potential can be reached.

It might be objected that in the experiment before shown the lamps,

being in multiple arc, the exhausted lamp might take a much larger

current and that the effect observed might not be exactly attributable

to the action of the gas in the bulbs. Such objections will lose much

weight if I connect the lamps in series, with the same result. When this

is done and the discharges are directed through the filaments, it is

again noted that the filament in the non-exhausted bulb l_{1}, remains

dark, while that in the exhausted one (_l_) glows even more intensely

than under its normal conditions of working, Fig. 186_b_. According to

general ideas the current through the filaments should now be the same,

were it not modified by the presence of the gas around the filaments.

At this juncture I may point out another interesting feature, which

illustrates the effect of the rate of change of potential of the

currents. I will leave the two lamps connected in series to the bars

B B_{1}, as in the previous experiment, Fig. 186_b_, but will presently

reduce considerably the frequency of the currents, which was excessive

in the experiment just before shown. This I may do by inserting a

self-induction coil in the path of the discharges, or by augmenting the

capacity of the condensers. When I now pass these low-frequency

discharges through the lamps, the exhausted lamp _l_ again is as bright

as before, but it is noted also that the non-exhausted lamp l_{1}

glows, though not quite as intensely as the other. Reducing the current

through the lamps, I may bring the filament in the latter lamp to

redness, and, though the filament in the exhausted lamp _l_ is bright,

Fig. 186_c_, the degree of its incandescence is much smaller than in

Fig. 186_b_, when the currents were of a much higher frequency.

In these experiments the gas acts in two opposite ways in determining

the degree of the incandescence of the filaments, that is, by convection

and bombardment. The higher the frequency and potential of the currents,

the more important becomes the bombardment. The convection on the

contrary should be the smaller, the higher the frequency. When the

currents are steady there is practically no bombardment, and convection

may therefore with such currents also considerably modify the degree of

incandescence and produce results similar to those just before shown.

Thus, if two lamps exactly alike, one exhausted and one not exhausted,

are connected in multiple arc or series to a direct-current machine, the

filament in the non-exhausted lamp will require a considerably greater

current to be rendered incandescent. This result is entirely due to

convection, and the effect is the more prominent the thinner the

filament. Professor Ayrton and Mr. Kilgour some time ago published

quantitative results concerning the thermal emissivity by radiation and

convection in which the effect with thin wires was clearly shown. This

effect may be strikingly illustrated by preparing a number of small,

short, glass tubes, each containing through its axis the thinnest

obtainable platinum wire. If these tubes be highly exhausted, a number

of them may be connected in multiple arc to a direct-current machine and

all of the wires may be kept at incandescence with a smaller current

than that required to render incandescent a single one of the wires if

the tube be not exhausted. Could the tubes be so highly exhausted that

convection would be nil, then the relative amounts of heat given off by

convection and radiation could be determined without the difficulties

attending thermal quantitative measurements. If a source of electric

impulses of high frequency and very high potential is employed, a still

greater number of the tubes may be taken and the wires rendered

incandescent by a current not capable of warming perceptibly a wire of

the same size immersed in air at ordinary pressure, and conveying the

energy to all of them.

I may here describe a result which is still more interesting, and to

which I have been led by the observation of these phenomena. I noted

that small differences in the density of the air produced a considerable

difference in the degree of incandescence of the wires, and I thought

that, since in a tube, through which a luminous discharge is passed, the

gas is generally not of uniform density, a very thin wire contained in

the tube might be rendered incandescent at certain places of smaller

density of the gas, while it would remain dark at the places of greater

density, where the convection would be greater and the bombardment less

intense. Accordingly a tube _t_ was prepared, as illustrated in Fig.

187, which contained through the middle a very fine platinum wire _w_.

The tube was exhausted to a moderate degree and it was found that when

it was attached to the terminal of a high-frequency coil the platinum

wire _w_ would indeed, become incandescent in patches, as illustrated in

Fig. 187. Later a number of these tubes with one or more wires were

prepared, each showing this result. The effect was best noted when the

striated discharge occurred in the tube, but was also produced when the

striae were not visible, showing that, even then, the gas in the tube was

not of uniform density. The position of the striae was generally such,

that the rarefactions corresponded to the places of incandescence or

greater brightness on the wire _w_. But in a few instances it was noted,

that the bright spots on the wire were covered by the dense parts of the

striated discharge as indicated by _l_ in Fig. 187, though the effect

was barely perceptible. This was explained in a plausible way by

assuming that the convection was not widely different in the dense and

rarefied places, and that the bombardment was greater on the dense

places of the striated discharge. It is, in fact, often observed in

bulbs, that under certain conditions a thin wire is brought to higher

incandescence when the air is not too highly rarefied. This is the case

when the potential of the coil is not high enough for the vacuum, but

the result may be attributed to many different causes. In all cases this

curious phenomenon of incandescence disappears when the tube, or rather

the wire, acquires throughout a uniform temperature.

[Illustration: FIG. 187.]

[Illustration: FIG. 188.]

Disregarding now the modifying effect of convection there are then two

distinct causes which determine the incandescence of a wire or filament

with varying currents, that is, conduction current and bombardment. With

steady currents we have to deal only with the former of these two

causes, and the heating effect is a minimum, since the resistance is

least to steady flow. When the current is a varying one the resistance

is greater, and hence the heating effect is increased. Thus if the rate

of change of the current is very great, the resistance may increase to

such an extent that the filament is brought to incandescence with

inappreciable currents, and we are able to take a short and thick block

of carbon or other material and bring it to bright incandescence with a

current incomparably smaller than that required to bring to the same

degree of incandescence an ordinary thin lamp filament with a steady or

low frequency current. This result is important, and illustrates how

rapidly our views on these subjects are changing, and how quickly our

field of knowledge is extending. In the art of incandescent lighting, to

view this result in one aspect only, it has been commonly considered as

an essential requirement for practical success, that the lamp filament

should be thin and of high resistance. But now we know that the

resistance of the filament to the steady flow does not mean anything;

the filament might as well be short and thick; for if it be immersed in

rarefied gas it will become incandescent by the passage of a small

current. It all depends on the frequency and potential of the currents.

We may conclude from this, that it would be of advantage, so far as the

lamp is considered, to employ high frequencies for lighting, as they

allow the use of short and thick filaments and smaller currents.

If a wire or filament be immersed in a homogeneous medium, all the

heating is due to true conduction current, but if it be enclosed in an

exhausted vessel the conditions are entirely different. Here the gas

begins to act and the heating effect of the conduction current, as is

shown in many experiments, may be very small compared with that of the

bombardment. This is especially the case if the circuit is not closed

and the potentials are of course very high. Suppose that a fine filament

enclosed in an exhausted vessel be connected with one of its ends to the

terminal of a high tension coil and with its other end to a large

insulated plate. Though the circuit is not closed, the filament, as I

have before shown, is brought to incandescence. If the frequency and

potential be comparatively low, the filament is heated by the current

passing _through it_. If the frequency and potential, and principally

the latter, be increased, the insulated plate need be but very small, or

may be done away with entirely; still the filament will become

incandescent, practically all the heating being then due to the

bombardment. A practical way of combining both the effects of conduction

currents and bombardment is illustrated in Fig. 188, in which an

ordinary lamp is shown provided with a very thin filament which has one

of the ends of the latter connected to a shade serving the purpose of

the insulated plate, and the other end to the terminal of a high tension

source. It should not be thought that only rarefied gas is an important

factor in the heating of a conductor by varying currents, but gas at

ordinary pressure may become important, if the potential difference and

frequency of the currents is excessive. On this subject I have already

stated, that when a conductor is fused by a stroke of lightning, the

current through it may be exceedingly small, not even sufficient to heat

the conductor perceptibly, were the latter immersed in a homogeneous

medium.

From the preceding it is clear that when a conductor of high resistance

is connected to the terminals of a source of high frequency currents of

high potential, there may occur considerable dissipation of energy,

principally at the ends of the conductor, in consequence of the action

of the gas surrounding the conductor. Owing to this, the current through

a section of the conductor at a point midway between its ends may be

much smaller than through a section near the ends. Furthermore, the

current passes principally through the outer portions of the conductor,

but this effect is to be distinguished from the skin effect as

ordinarily interpreted, for the latter would, or should, occur also in a

continuous incompressible medium. If a great many incandescent lamps are

connected in series to a source of such currents, the lamps at the ends

may burn brightly, whereas those in the middle may remain entirely dark.

This is due principally to bombardment, as before stated. But even if

the currents be steady, provided the difference of potential is very

great, the lamps at the end will burn more brightly than those in the

middle. In such case there is no rhythmical bombardment, and the result

is produced entirely by leakage. This leakage or dissipation into space

when the tension is high, is considerable when incandescent lamps are

used, and still more considerable with arcs, for the latter act like

flames. Generally, of course, the dissipation is much smaller with

steady, than with varying, currents.

I have contrived an experiment which illustrates in an interesting

manner the effect of lateral diffusion. If a very long tube is attached

to the terminal of a high frequency coil, the luminosity is greatest

near the terminal and falls off gradually towards the remote end. This

is more marked if the tube is narrow.

A small tube about one-half inch in diameter and twelve inches long

(Fig. 189), has one of its ends drawn out into a fine fibre _f_ nearly

three feet long. The tube is placed in a brass socket T which can be

screwed on the terminal T_{1} of the induction coil. The discharge

passing through the tube first illuminates the bottom of the same, which

is of comparatively large section; but through the long glass fibre the

discharge cannot pass. But gradually the rarefied gas inside becomes

warmed and more conducting and the discharge spreads into the glass

fibre. This spreading is so slow, that it may take half a minute or more

until the discharge has worked through up to the top of the glass fibre,

then presenting the appearance of a strongly luminous thin thread. By

adjusting the potential at the terminal the light may be made to travel

upwards at any speed. Once, however, the glass fibre is heated, the

discharge breaks through its entire length instantly. The interesting

point to be noted is that, the higher the frequency of the currents, or

in other words, the greater relatively the lateral dissipation, at a

slower rate may the light be made to propagate through the fibre. This

experiment is best performed with a highly exhausted and freshly made

tube. When the tube has been used for some time the experiment often

fails. It is possible that the gradual and slow impairment of the vacuum

is the cause. This slow propagation of the discharge through a very

narrow glass tube corresponds exactly to the propagation of heat through

a bar warmed at one end. The quicker the heat is carried away laterally

the longer time it will take for the heat to warm the remote end. When

the current of a low frequency coil is passed through the fibre from end

to end, then the lateral dissipation is small and the discharge

instantly breaks through almost without exception.

[Illustration: FIG. 189.]

[Illustration: FIG. 190.]

After these experiments and observations which have shown the importance

of the discontinuity or atomic structure of the medium and which will

serve to explain, in a measure at least, the nature of the four kinds of

light effects producible with these currents, I may now give you an

illustration of these effects. For the sake of interest I may do this in

a manner which to many of you might be novel. You have seen before that

we may now convey the electric vibration to a body by means of a single

wire or conductor of any kind. Since the human frame is conducting I

may convey the vibration through my body.

First, as in some previous experiments, I connect my body with one of

the terminals of a high-tension transformer and take in my hand an

exhausted bulb which contains a small carbon button mounted upon a

platinum wire leading to the outside of the bulb, and the button is

rendered incandescent as soon as the transformer is set to work (Fig.

190). I may place a conducting shade on the bulb which serves to

intensify the action, but is not necessary. Nor is it required that the

button should be in conducting connection with the hand through a wire

leading through the glass, for sufficient energy may be transmitted

through the glass itself by inductive action to render the button

incandescent.

[Illustration: FIG. 191.]

[Illustration: FIG. 192.]

Next I take a highly exhausted bulb containing a strongly phosphorescent

body, above which is mounted a small plate of aluminum on a platinum

wire leading to the outside, and the currents flowing through my body

excite intense phosphorescence in the bulb (Fig. 191). Next again I take

in my hand a simple exhausted tube, and in the same manner the gas

inside the tube is rendered highly incandescent or phosphorescent (Fig.

192). Finally, I may take in my hand a wire, bare or covered with thick

insulation, it is quite immaterial; the electrical vibration is so

intense as to cover the wire with a luminous film (Fig. 193).

[Illustration: FIG. 193.]

[Illustration: FIG. 194.]

[Illustration: FIG. 195.]

A few words must now be devoted to each of these phenomena. In the first

place, I will consider the incandescence of a button or of a solid in

general, and dwell upon some facts which apply equally to all these

phenomena. It was pointed out before that when a thin conductor, such as

a lamp filament, for instance, is connected with one of its ends to the

terminal of a transformer of high tension the filament is brought to

incandescence partly by a conduction current and partly by bombardment.

The shorter and thicker the filament the more important becomes the

latter, and finally, reducing the filament to a mere button, all the

heating must practically be attributed to the bombardment. So in the

experiment before shown, the button is rendered incandescent by the

rhythmical impact of freely movable small bodies in the bulb. These

bodies may be the molecules of the residual gas, particles of dust or

lumps torn from the electrode; whatever they are, it is certain that the

heating of the button is essentially connected with the pressure of such

freely movable particles, or of atomic matter in general in the bulb.

The heating is the more intense the greater the number of impacts per

second and the greater the energy of each impact. Yet the button would

be heated also if it were connected to a source of a steady potential.

In such a case electricity would be carried away from the button by the

freely movable carriers or particles flying about, and the quantity of

electricity thus carried away might be sufficient to bring the button to

incandescence by its passage through the latter. But the bombardment

could not be of great importance in such case. For this reason it would

require a comparatively very great supply of energy to the button to

maintain it at incandescence with a steady potential. The higher the

frequency of the electric impulses the more economically can the button

be maintained at incandescence. One of the chief reasons why this is so,

is, I believe, that with impulses of very high frequency there is less

exchange of the freely movable carriers around the electrode and this

means, that in the bulb the heated matter is better confined to the

neighborhood of the button. If a double bulb, as illustrated in Fig. 194

be made, comprising a large globe B and a small one _b_, each containing

as usual a filament _f_ mounted on a platinum wire w and w_{1}, it

is found, that if the filaments _f f_ be exactly alike, it requires less

energy to keep the filament in the globe _b_ at a certain degree of

incandescence, than that in the globe B. This is due to the confinement

of the movable particles around the button. In this case it is also

ascertained, that the filament in the small globe _b_ is less

deteriorated when maintained a certain length of time at incandescence.

This is a necessary consequence of the fact that the gas in the small

bulb becomes strongly heated and therefore a very good conductor, and

less work is then performed on the button, since the bombardment becomes

less intense as the conductivity of the gas increases. In this

construction, of course, the small bulb becomes very hot and when it

reaches an elevated temperature the convection and radiation on the

outside increase. On another occasion I have shown bulbs in which this

drawback was largely avoided. In these instances a very small bulb,

containing a refractory button, was mounted in a large globe and the

space between the walls of both was highly exhausted. The outer large

globe remained comparatively cool in such constructions. When the large

globe was on the pump and the vacuum between the walls maintained

permanent by the continuous action of the pump, the outer globe would

remain quite cold, while the button in the small bulb was kept at

incandescence. But when the seal was made, and the button in the small

bulb maintained incandescent some length of time, the large globe too

would become warmed. From this I conjecture that if vacuous space (as

Prof. Dewar finds) cannot convey heat, it is so merely in virtue of our

rapid motion through space or, generally speaking, by the motion of the

medium relatively to us, for a permanent condition could not be

maintained without the medium being constantly renewed. A vacuum cannot,

according to all evidence, be permanently maintained around a hot body.

In these constructions, before mentioned, the small bulb inside would,

at least in the first stages, prevent all bombardment against the outer

large globe. It occurred to me then to ascertain how a metal sieve would

behave in this respect, and several bulbs, as illustrated in Fig. 195,

were prepared for this purpose. In a globe _b_, was mounted a thin

filament _f_ (or button) upon a platinum wire _w_ passing through a

glass stem and leading to the outside of the globe. The filament _f_ was

surrounded by a metal sieve _s_. It was found in experiments with such

bulbs that a sieve with wide meshes apparently did not in the slightest

affect the bombardment against the globe _b_. When the vacuum was high,

the shadow of the sieve was clearly projected against the globe and the

latter would get hot in a short while. In some bulbs the sieve _s_ was

connected to a platinum wire sealed in the glass. When this wire was

connected to the other terminal of the induction coil (the E. M. F.

being kept low in this case), or to an insulated plate, the bombardment

against the outer globe _b_ was diminished. By taking a sieve with fine

meshes the bombardment against the globe _b_ was always diminished, but

even then if the exhaustion was carried very far, and when the potential

of the transformer was very high, the globe _b_ would be bombarded and

heated quickly, though no shadow of the sieve was visible, owing to the

smallness of the meshes. But a glass tube or other continuous body

mounted so as to surround the filament, did entirely cut off the

bombardment and for a while the outer globe _b_ would remain perfectly

cold. Of course when the glass tube was sufficiently heated the

bombardment against the outer globe could be noted at once. The

experiments with these bulbs seemed to show that the speeds of the

projected molecules or particles must be considerable (though quite

insignificant when compared with that of light), otherwise it would be

difficult to understand how they could traverse a fine metal sieve

without being affected, unless it were found that such small particles

or atoms cannot be acted upon directly at measurable distances. In

regard to the speed of the projected atoms, Lord Kelvin has recently

estimated it at about one kilometre a second or thereabouts in an

ordinary Crookes bulb. As the potentials obtainable with a disruptive

discharge coil are much higher than with ordinary coils, the speeds

must, of course, be much greater when the bulbs are lighted from such a

coil. Assuming the speed to be as high as five kilometres and uniform

through the whole trajectory, as it should be in a very highly exhausted

vessel, then if the alternate electrifications of the electrode would be

of a frequency of five million, the greatest distance a particle could

get away from the electrode would be one millimetre, and if it could be

acted upon directly at that distance, the exchange of electrode matter

or of the atoms would be very slow and there would be practically no

bombardment against the bulb. This at least should be so, if the action

of an electrode upon the atoms of the residual gas would be such as upon

electrified bodies which we can perceive. A hot body enclosed in an

exhausted bulb produces always atomic bombardment, but a hot body has no

definite rhythm, for its molecules perform vibrations of all kinds.

If a bulb containing a button or filament be exhausted as high as is

possible with the greatest care and by the use of the best artifices, it

is often observed that the discharge cannot, at first, break through,

but after some time, probably in consequence of some changes within the

bulb, the discharge finally passes through and the button is rendered

incandescent. In fact, it appears that the higher the degree of

exhaustion the easier is the incandescence produced. There seem to be no

other causes to which the incandescence might be attributed in such case

except to the bombardment or similar action of the residual gas, or of

particles of matter in general. But if the bulb be exhausted with the

greatest care can these play an important part? Assume the vacuum in the

bulb to be tolerably perfect, the great interest then centres in the

question: Is the medium which pervades all space continuous or atomic?

If atomic, then the heating of a conducting button or filament in an

exhausted vessel might be due largely to ether bombardment, and then the

heating of a conductor in general through which currents of high

frequency or high potential are passed must be modified by the behavior

of such medium; then also the skin effect, the apparent increase of the

ohmic resistance, etc., admit, partially at least, of a different

explanation.

It is certainly more in accordance with many phenomena observed with

high-frequency currents to hold that all space is pervaded with free

atoms, rather than to assume that it is devoid of these, and dark and

cold, for so it must be, if filled with a continuous medium, since in

such there can be neither heat nor light. Is then energy transmitted by

independent carriers or by the vibration of a continuous medium? This

important question is by no means as yet positively answered. But most

of the effects which are here considered, especially the light effects,

incandescence, or phosphorescence, involve the presence of free atoms

and would be impossible without these.

In regard to the incandescence of a refractory button (or filament) in

an exhausted receiver, which has been one of the subjects of this

investigation, the chief experiences, which may serve as a guide in

constructing such bulbs, may be summed up as follows: 1. The button

should be as small as possible, spherical, of a smooth or polished

surface, and of refractory material which withstands evaporation best.

2. The support of the button should be very thin and screened by an

aluminum and mica sheet, as I have described on another occasion. 3. The

exhaustion of the bulb should be as high as possible. 4. The frequency

of the currents should be as high as practicable. 5. The currents should

be of a harmonic rise and fall, without sudden interruptions. 6. The

heat should be confined to the button by inclosing the same in a small

bulb or otherwise. 7. The space between the walls of the small bulb and

the outer globe should be highly exhausted.

Most of the considerations which apply to the incandescence of a solid

just considered may likewise be applied to phosphorescence. Indeed, in

an exhausted vessel the phosphorescence is, as a rule, primarily excited

by the powerful beating of the electrode stream of atoms against the

phosphorescent body. Even in many cases, where there is no evidence of

such a bombardment, I think that phosphorescence is excited by violent

impacts of atoms, which are not necessarily thrown off from the

electrode but are acted upon from the same inductively through the

medium or through chains of other atoms. That mechanical shocks play an

important part in exciting phosphorescence in a bulb may be seen from

the following experiment. If a bulb, constructed as that illustrated in

Fig. 174, be taken and exhausted with the greatest care so that the

discharge cannot pass, the filament _f_ acts by electrostatic induction

upon the tube _t_ and the latter is set in vibration. If the tube _o_ be

rather wide, about an inch or so, the filament may be so powerfully

vibrated that whenever it hits the glass tube it excites

phosphorescence. But the phosphorescence ceases when the filament comes

to rest. The vibration can be arrested and again started by varying the

frequency of the currents. Now the filament has its own period of

vibration, and if the frequency of the currents is such that there is

resonance, it is easily set vibrating, though the potential of the

currents be small. I have often observed that the filament in the bulb

is destroyed by such mechanical resonance. The filament vibrates as a

rule so rapidly that it cannot be seen and the experimenter may at first

be mystified. When such an experiment as the one described is carefully

performed, the potential of the currents need be extremely small, and

for this reason I infer that the phosphorescence is then due to the

mechanical shock of the filament against the glass, just as it is

produced by striking a loaf of sugar with a knife. The mechanical shock

produced by the projected atoms is easily noted when a bulb containing a

button is grasped in the hand and the current turned on suddenly. I

believe that a bulb could be shattered by observing the conditions of

resonance.

In the experiment before cited it is, of course, open to say, that the

glass tube, upon coming in contact with the filament, retains a charge

of a certain sign upon the point of contact. If now the filament again

touches the glass at the same point while it is oppositely charged, the

charges equalize under evolution of light. But nothing of importance

would be gained by such an explanation. It is unquestionable that the

initial charges given to the atoms or to the glass play some part in

exciting phosphorescence. So, for instance, if a phosphorescent bulb be

first excited by a high frequency coil by connecting it to one of the

terminals of the latter and the degree of luminosity be noted, and then

the bulb be highly charged from a Holtz machine by attaching it

preferably to the positive terminal of the machine, it is found that

when the bulb is again connected to the terminal of the high frequency

coil, the phosphorescence is far more intense. On another occasion I

have considered the possibility of some phosphorescent phenomena in

bulbs being produced by the incandescence of an infinitesimal layer on

the surface of the phosphorescent body. Certainly the impact of the

atoms is powerful enough to produce intense incandescence by the

collisions, since they bring quickly to a high temperature a body of

considerable bulk. If any such effect exists, then the best appliance

for producing phosphorescence in a bulb, which we know so far, is a

disruptive discharge coil giving an enormous potential with but few

fundamental discharges, say 25-30 per second, just enough to produce a

continuous impression upon the eye. It is a fact that such a coil

excites phosphorescence under almost any condition and at all degrees of

exhaustion, and I have observed effects which appear to be due to

phosphorescence even at ordinary pressures of the atmosphere, when the

potentials are extremely high. But if phosphorescent light is produced

by the equalization of charges of electrified atoms (whatever this may

mean ultimately), then the higher the frequency of the impulses or

alternate electrifications, the more economical will be the light

production. It is a long known and noteworthy fact that all the

phosphorescent bodies are poor conductors of electricity and heat, and

that all bodies cease to emit phosphorescent light when they are brought

to a certain temperature. Conductors on the contrary do not possess this

quality. There are but few exceptions to the rule. Carbon is one of

them. Becquerel noted that carbon phosphoresces at a certain elevated

temperature preceding the dark red. This phenomenon may be easily

observed in bulbs provided with a rather large carbon electrode (say, a

sphere of six millimetres diameter). If the current is turned on after a

few seconds, a snow white film covers the electrode, just before it gets

dark red. Similar effects are noted with other conducting bodies, but

many scientific men will probably not attribute them to true

phosphorescence. Whether true incandescence has anything to do with

phosphorescence excited by atomic impact or mechanical shocks still

remains to be decided, but it is a fact that all conditions, which tend

to localize and increase the heating effect at the point of impact, are

almost invariably the most favorable for the production of

phosphorescence. So, if the electrode be very small, which is equivalent

to saying in general, that the electric density is great; if the

potential be high, and if the gas be highly rarefied, all of which

things imply high speed of the projected atoms, or matter, and

consequently violent impacts--the phosphorescence is very intense. If a

bulb provided with a large and small electrode be attached to the

terminal of an induction coil, the small electrode excites

phosphorescence while the large one may not do so, because of the

smaller electric density and hence smaller speed of the atoms. A bulb

provided with a large electrode may be grasped with the hand while the

electrode is connected to the terminal of the coil and it may not

phosphoresce; but if instead of grasping the bulb with the hand, the

same be touched with a pointed wire, the phosphorescence at once

spreads through the bulb, because of the great density at the point of

contact. With low frequencies it seems that gases of great atomic weight

excite more intense phosphorescence than those of smaller weight, as for

instance, hydrogen. With high frequencies the observations are not

sufficiently reliable to draw a conclusion. Oxygen, as is well-known,

produces exceptionally strong effects, which may be in part due to

chemical action. A bulb with hydrogen residue seems to be most easily

excited. Electrodes which are most easily deteriorated produce more

intense phosphorescence in bulbs, but the condition is not permanent

because of the impairment of the vacuum and the deposition of the

electrode matter upon the phosphorescent surfaces. Some liquids, as

oils, for instance, produce magnificent effects of phosphorescence (or

fluorescence?), but they last only a few seconds. So if a bulb has a

trace of oil on the walls and the current is turned on, the

phosphorescence only persists for a few moments until the oil is carried

away. Of all bodies so far tried, sulphide of zinc seems to be the most

susceptible to phosphorescence. Some samples, obtained through the

kindness of Prof. Henry in Paris, were employed in many of these bulbs.

One of the defects of this sulphide is, that it loses its quality of

emitting light when brought to a temperature which is by no means high.

It can therefore, be used only for feeble intensities. An observation

which might deserve notice is, that when violently bombarded from an

aluminum electrode it assumes a black color, but singularly enough, it

returns to the original condition when it cools down.

The most important fact arrived at in pursuing investigations in this

direction is, that in all cases it is necessary, in order to excite

phosphorescence with a minimum amount of energy, to observe certain

conditions. Namely, there is always, no matter what the frequency of the

currents, degree of exhaustion and character of the bodies in the bulb,

a certain potential (assuming the bulb excited from one terminal) or

potential difference (assuming the bulb to be excited with both

terminals) which produces the most economical result. If the potential

be increased, considerable energy may be wasted without producing any

more light, and if it be diminished, then again the light production is

not as economical. The exact condition under which the best result is

obtained seems to depend on many things of a different nature, and it is

to be yet investigated by other experimenters, but it will certainly

have to be observed when such phosphorescent bulbs are operated, if the

best results are to be obtained.

Coming now to the most interesting of these phenomena, the incandescence

or phosphorescence of gases, at low pressures or at the ordinary

pressure of the atmosphere, we must seek the explanation of these

phenomena in the same primary causes, that is, in shocks or impacts of

the atoms. Just as molecules or atoms beating upon a solid body excite

phosphorescence in the same or render it incandescent, so when colliding

among themselves they produce similar phenomena. But this is a very

insufficient explanation and concerns only the crude mechanism. Light is

produced by vibrations which go on at a rate almost inconceivable. If we

compute, from the energy contained in the form of known radiations in a

definite space the force which is necessary to set up such rapid

vibrations, we find, that though the density of the ether be

incomparably smaller than that of any body we know, even hydrogen, the

force is something surpassing comprehension. What is this force, which

in mechanical measure may amount to thousands of tons per square inch?

It is electrostatic force in the light of modern views. It is impossible

to conceive how a body of measurable dimensions could be charged to so

high a potential that the force would be sufficient to produce these

vibrations. Long before any such charge could be imparted to the body it

would be shattered into atoms. The sun emits light and heat, and so does

an ordinary flame or incandescent filament, but in neither of these can

the force be accounted for if it be assumed that it is associated with

the body as a whole. Only in one way may we account for it, namely, by

identifying it with the atom. An atom is so small, that if it be charged

by coming in contact with an electrified body and the charge be assumed

to follow the same law as in the case of bodies of measurable

dimensions, it must retain a quantity of electricity which is fully

capable of accounting for these forces and tremendous rates of

vibration. But the atom behaves singularly in this respect--it always

takes the same "charge."

It is very likely that resonant vibration plays a most important part in

all manifestations of energy in nature. Throughout space all matter is

vibrating, and all rates of vibration are represented, from the lowest

musical note to the highest pitch of the chemical rays, hence an atom,

or complex of atoms, no matter what its period, must find a vibration

with which it is in resonance. When we consider the enormous rapidity

of the light vibrations, we realize the impossibility of producing such

vibrations directly with any apparatus of measurable dimensions, and we

are driven to the only possible means of attaining the object of setting

up waves of light by electrical means and economically, that is, to

affect the molecules or atoms of a gas, to cause them to collide and

vibrate. We then must ask ourselves--How can free molecules or atoms be

affected?

[Illustration: FIG. 196.]

[Illustration: FIG. 197.]

It is a fact that they can be affected by electrostatic force, as is

apparent in many of these experiments. By varying the electrostatic

force we can agitate the atoms, and cause them to collide accompanied by

evolution of heat and light. It is not demonstrated beyond doubt that we

can affect them otherwise. If a luminous discharge is produced in a

closed exhausted tube, do the atoms arrange themselves in obedience to

any other but to electrostatic force acting in straight lines from atom

to atom? Only recently I investigated the mutual action between two

circuits with extreme rates of vibration. When a battery of a few jars

(_c c c c_, Fig. 196) is discharged through a primary P of low

resistance (the connections being as illustrated in Figs. 183_a_, 183_b_

and 183_c_), and the frequency of vibration is many millions there are

great differences of potential between points on the primary not more

than a few inches apart. These differences may be 10,000 volts per inch,

if not more, taking the maximum value of the E. M. F. The secondary _s_

is therefore acted upon by electrostatic induction, which is in such

extreme cases of much greater importance than the electro-dynamic. To

such sudden impulses the primary as well as the secondary are poor

conductors, and therefore great differences of potential may be produced

by electrostatic induction between adjacent points on the secondary.

Then sparks may jump between the wires and streamers become visible in

the dark if the light of the discharge through the spark gap _d d_ be

carefully excluded. If now we substitute a closed vacuum tube for the

metallic secondary _s_, the differences of potential produced in the

tube by electrostatic induction from the primary are fully sufficient to

excite portions of it; but as the points of certain differences of

potential on the primary are not fixed, but are generally constantly

changing in position, a luminous band is produced in the tube,

apparently not touching the glass, as it should, if the points of

maximum and minimum differences of potential were fixed on the primary.

I do not exclude the possibility of such a tube being excited only by

electro-dynamic induction, for very able physicists hold this view; but

in my opinion, there is as yet no positive proof given that atoms of a

gas in a closed tube may arrange themselves in chains under the action

of an electromotive impulse produced by electro-dynamic induction in the

tube. I have been unable so far to produce striae in a tube, however

long, and at whatever degree of exhaustion, that is, striae at right

angles to the supposed direction of the discharge or the axis of the

tube; but I have distinctly observed in a large bulb, in which a wide

luminous band was produced by passing a discharge of a battery through a

wire surrounding the bulb, a circle of feeble luminosity between two

luminous bands, one of which was more intense than the other.

Furthermore, with my present experience I do not think that such a gas

discharge in a closed tube can vibrate, that is, vibrate as a whole. I

am convinced that no discharge through a gas can vibrate. The atoms of a

gas behave very curiously in respect to sudden electric impulses. The

gas does not seem to possess any appreciable inertia to such impulses,

for it is a fact, that the higher the frequency of the impulses, with

the greater freedom does the discharge pass through the gas. If the gas

possesses no inertia then it cannot vibrate, for some inertia is

necessary for the free vibration. I conclude from this that if a

lightning discharge occurs between two clouds, there can be no

oscillation, such as would be expected, considering the capacity of the

clouds. But if the lightning discharge strike the earth, there is always

vibration--in the earth, but not in the cloud. In a gas discharge each

atom vibrates at its own rate, but there is no vibration of the

conducting gaseous mass as a whole. This is an important consideration

in the great problem of producing light economically, for it teaches us

that to reach this result we must use impulses of very high frequency

and necessarily also of high potential. It is a fact that oxygen

produces a more intense light in a tube. Is it because oxygen atoms

possess some inertia and the vibration does not die out instantly? But

then nitrogen should be as good, and chlorine and vapors of many other

bodies much better than oxygen, unless the magnetic properties of the

latter enter prominently into play. Or, is the process in the tube of an

electrolytic nature? Many observations certainly speak for it, the most

important being that matter is always carried away from the electrodes

and the vacuum in a bulb cannot be permanently maintained. If such

process takes place in reality, then again must we take refuge in high

frequencies, for, with such, electrolytic action should be reduced to a

minimum, if not rendered entirely impossible. It is an undeniable fact

that with very high frequencies, provided the impulses be of harmonic

nature, like those obtained from an alternator, there is less

deterioration and the vacua are more permanent. With disruptive

discharge coils there are sudden rises of potential and the vacua are

more quickly impaired, for the electrodes are deteriorated in a very

short time. It was observed in some large tubes, which were provided

with heavy carbon blocks B B_{1}, connected to platinum wires w w_{1}

(as illustrated in Fig. 197), and which were employed in experiments

with the disruptive discharge instead of the ordinary air gap, that the

carbon particles under the action of the powerful magnetic field in

which the tube was placed, were deposited in regular fine lines in the

middle of the tube, as illustrated. These lines were attributed to the

deflection or distortion of the discharge by the magnetic field, but why

the deposit occurred principally where the field was most intense did

not appear quite clear. A fact of interest, likewise noted, was that the

presence of a strong magnetic field increases the deterioration of the

electrodes, probably by reason of the rapid interruptions it produces,

whereby there is actually a higher E. M. F. maintained between the

electrodes.

Much would remain to be said about the luminous effects produced in

gases at low or ordinary pressures. With the present experiences before

us we cannot say that the essential nature of these charming phenomena

is sufficiently known. But investigations in this direction are being

pushed with exceptional ardor. Every line of scientific pursuit has its

fascinations, but electrical investigation appears to possess a

peculiar attraction, for there is no experiment or observation of any

kind in the domain of this wonderful science which would not forcibly

appeal to us. Yet to me it seems, that of all the many marvelous things

we observe, a vacuum tube, excited by an electric impulse from a distant

source, bursting forth out of the darkness and illuminating the room

with its beautiful light, is as lovely a phenomenon as can greet our

eyes. More interesting still it appears when, reducing the fundamental

discharges across the gap to a very small number and waving the tube

about we produce all kinds of designs in luminous lines. So by way of

amusement I take a straight long tube, or a square one, or a square

attached to a straight tube, and by whirling them about in the hand, I

imitate the spokes of a wheel, a Gramme winding, a drum winding, an

alternate current motor winding, etc. (Fig. 198). Viewed from a distance

the effect is weak and much of its beauty is lost, but being near or

holding the tube in the hand, one cannot resist its charm.

[Illustration: FIG. 198.]

In presenting these insignificant results I have not attempted to

arrange and co-ordinate them, as would be proper in a strictly

scientific investigation, in which every succeeding result should be a

logical sequence of the preceding, so that it might be guessed in

advance by the careful reader or attentive listener. I have preferred to

concentrate my energies chiefly upon advancing novel facts or ideas

which might serve as suggestions to others, and this may serve as an

excuse for the lack of harmony. The explanations of the phenomena have

been given in good faith and in the spirit of a student prepared to find

that they admit of a better interpretation. There can be no great harm

in a student taking an erroneous view, but when great minds err, the

world must dearly pay for their mistakes.

CHAPTER XXIX.

TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY, IN DETAIL.

It has become a common practice to operate arc lamps by alternating or

pulsating, as distinguished from continuous, currents; but an objection

which has been raised to such systems exists in the fact that the arcs

emit a pronounced sound, varying with the rate of the alternations or

pulsations of current. This noise is due to the rapidly alternating

heating and cooling, and consequent expansion and contraction, of the

gaseous matter forming the arc, which corresponds with the periods or

impulses of the current. Another disadvantageous feature is found in the

difficulty of maintaining an alternating current arc in consequence of

the periodical increase in resistance corresponding to the periodical

working of the current. This feature entails a further disadvantage,

namely, that small arcs are impracticable.

Theoretical considerations have led Mr. Tesla to the belief that these

disadvantageous features could be obviated by employing currents of a

sufficiently high number of alternations, and his anticipations have

been confirmed in practice. These rapidly alternating currents render it

possible to maintain small arcs which, besides, possess the advantages

of silence and persistency. The latter quality is due to the necessarily

rapid alternations, in consequence of which the arc has no time to cool,

and is always maintained at a high temperature and low resistance.

At the outset of his experiments Mr. Tesla encountered great

difficulties in the construction of high frequency machines. A generator

of this kind is described here, which, though constructed quite some

time ago, is well worthy of a detailed description. It may be mentioned,

in passing, that dynamos of this type have been used by Mr. Tesla in his

lighting researches and experiments with currents of high potential and

high frequency, and reference to them will be found in his lectures

elsewhere printed in this volume.[4]

[4] See pages 153-4 5.

In the accompanying engravings, Figs. 199 and 200 show the machine,

respectively, in side elevation and vertical cross-section; Figs. 201,

202 and 203 showing enlarged details of construction. As will be seen, A

is an annular magnetic frame, the interior of which is provided with a

large number of pole-pieces D.

Owing to the very large number and small size of the poles and the

spaces between them, the field coils are applied by winding an insulated

conductor F zigzag through the grooves, as shown in Fig. 203, carrying

the wire around the annulus to form as many layers as is desired. In

this way the pole-pieces D will be energized with alternately opposite

polarity around the entire ring.

For the armature, Mr. Tesla employs a spider carrying a ring J, turned

down, except at its edges, to form a trough-like receptacle for a mass

of fine annealed iron wires K, which are wound in the groove to form the

core proper for the armature-coils. Pins L are set in the sides of the

ring J and the coils M are wound over the periphery of the

armature-structure and around the pins. The coils M are connected

together in series, and these terminals N carried through the hollow

shaft H to contact-rings P P, from which the currents are taken off by

brushes O.

[Illustration: FIG. 199.]

In this way a machine with a very large number of poles may be

constructed. It is easy, for instance, to obtain in this manner three

hundred and seventy-five to four hundred poles in a machine that may be

safely driven at a speed of fifteen hundred or sixteen hundred

revolutions per minute, which will produce ten thousand or eleven

thousand alternations of current per second. Arc lamps R R are shown in

the diagram as connected up in series with the machine in Fig. 200. If

such a current be applied to running arc lamps, the sound produced by or

in the arc becomes practically inaudible, for, by increasing the rate of

change in the current, and consequently the number of vibrations per

unit of time of the gaseous material of the arc up to, or beyond, ten

thousand or eleven thousand per second, or to what is regarded as the

limit of audition, the sound due to such vibrations will not be audible.

The exact number of changes or undulations necessary to produce this

result will vary somewhat according to the size of the arc--that is to

say, the smaller the arc, the greater the number of changes that will be

required to render it inaudible within certain limits. It should also be

stated that the arc should not exceed a certain length.

[Illustration: FIGS. 200, 201, 202 and 203.]

The difficulties encountered in the construction of these machines are

of a mechanical as well as an electrical nature. The machines may be

designed on two plans: the field may be formed either of alternating

poles, or of polar projections of the same polarity. Up to about 15,000

alternations per second in an experimental machine, the former plan may

be followed, but a more efficient machine is obtained on the second

plan.

In the machine above described, which was capable of running two arcs of

normal candle power, the field was composed of a ring of wrought iron

32 inches outside diameter, and about 1 inch thick. The inside diameter

was 30 inches. There were 384 polar projections. The wire was wound in

zigzag form, but two wires were wound so as to completely envelop the

projections. The distance between the projections is about 3/16 inch,

and they are a little over 1/16 inch thick. The field magnet was made

relatively small so as to adapt the machine for a constant current.

There are 384 coils connected in two series. It was found impracticable

to use any wire much thicker than No. 26 B. and S. gauge on account of

the local effects. In such a machine the clearance should be as small as

possible; for this reason the machine was made only 1-1/4 inch wide, so

that the binding wires might be obviated. The armature wires must be

wound with great care, as they are apt to fly off in consequence of the

great peripheral speed. In various experiments this machine has been run

as high as 3,000 revolutions per minute. Owing to the great speed it was

possible to obtain as high as 10 amperes out of the machine. The

electromotive force was regulated by means of an adjustable condenser

within very wide limits, the limits being the greater, the greater the

speed. This machine was frequently used to run Mr. Tesla's laboratory

lights.

[Illustration: FIG. 204.]

The machine above described was only one of many such types constructed.

It serves well for an experimental machine, but if still higher

alternations are required and higher efficiency is necessary, then a

machine on a plan shown in Figs. 204 to 207, is preferable. The

principal advantage of this type of machine is that there is not much

magnetic leakage, and that a field may be produced, varying greatly in

intensity in places not much distant from each other.

In these engravings, Figs. 204 and 205 illustrate a machine in which the

armature conductor and field coils are stationary, while the field

magnet core revolves. Fig. 206 shows a machine embodying the same plan

of construction, but having a stationary field magnet and rotary

armature.

The conductor in which the currents are induced may be arranged in

various ways; but Mr. Tesla prefers the following method: He employs an

annular plate of copper D, and by means of a saw cuts in it radial slots

from one edge nearly through to the other, beginning alternately from

opposite edges. In this way a continuous zigzag conductor is formed.

When the polar projections are 1/8 inch wide, the width of the conductor

should not, under any circumstances, be more than 1/32 inch wide; even

then the eddy effect is considerable.

[Illustration: FIG. 205.]

To the inner edge of this plate are secured two rings of non-magnetic

metal E, which are insulated from the copper conductor, but held firmly

thereto by means of the bolts F. Within the rings E is then placed an

annular coil G, which is the energizing coil for the field magnet. The

conductor D and the parts attached thereto are supported by means of the

cylindrical shell or casting A A, the two parts of which are brought

together and clamped to the outer edge of the conductor D.

[Illustration: FIG. 206.]

The core for the field magnet is built up of two circular parts H H,

formed with annular grooves I, which, when the two parts are brought

together, form a space for the reception of the energizing coil G. The

hubs of the cores are trued off, so as to fit closely against one

another, while the outer portions or flanges which form the polar faces

J J, are reduced somewhat in thickness to make room for the conductor D,

and are serrated on their faces. The number of serrations in the polar

faces is arbitrary; but there must exist between them and the radial

portions of the conductor D certain relation, which will be understood

by reference to Fig. 207 in which N N represent the projections or

points on one face of the core of the field, and S S the points of the

other face. The conductor D is shown in this figure in section _a a'_

designating the radial portions of the conductor, and _b_ the insulating

divisions between them. The relative width of the parts _a a'_ and the

space between any two adjacent points N N or S S is such that when the

radial portions _a_ of the conductor are passing between the opposite

points N S where the field is strongest, the intermediate radial

portions _a'_ are passing through the widest spaces midway between such

points and where the field is weakest. Since the core on one side is of

opposite polarity to the part facing it, all the projections of one

polar face will be of opposite polarity to those of the other face.

Hence, although the space between any two adjacent points on the same

face may be extremely small, there will be no leakage of the magnetic

lines between any two points of the same name, but the lines of force

will pass across from one set of points to the other. The construction

followed obviates to a great degree the distortion of the magnetic lines

by the action of the current in the conductor D, in which it will be

observed the current is flowing at any given time from the centre toward

the periphery in one set of radial parts _a_ and in the opposite

direction in the adjacent parts _a'_.

In order to connect the energizing coil G, Fig. 204, with a source of

continuous current, Mr. Tesla utilizes two adjacent radial portions of

the conductor D for connecting the terminals of the coil G with two

binding posts M. For this purpose the plate D is cut entirely through,

as shown, and the break thus made is bridged over by a short conductor

C. The plate D is cut through to form two terminals _d_, which are

connected to binding posts N. The core H H, when rotated by the driving

pulley, generates in the conductors D an alternating current, which is

taken off from the binding posts N.

[Illustration: FIG. 207.]

When it is desired to rotate the conductor between the faces of a

stationary field magnet, the construction shown in Fig. 206, is adopted.

The conductor D in this case is or may be made in substantially the same

manner as above described by slotting an annular conducting-plate and

supporting it between two heads O, held together by bolts _o_ and fixed

to the driving-shaft K. The inner edge of the plate or conductor D is

preferably flanged to secure a firmer union between it and the heads O.

It is insulated from the head. The field-magnet in this case consists of

two annular parts H H, provided with annular grooves I for the reception

of the coils. The flanges or faces surrounding the annular groove are

brought together, while the inner flanges are serrated, as in the

previous case, and form the polar faces. The two parts H H are formed

with a base R, upon which the machine rests. S S are non-magnetic

bushings secured or set in the central opening of the cores. The

conductor D is cut entirely through at one point to form terminals, from

which insulated conductors T are led through the shaft to

collecting-rings V.

In one type of machine of this kind constructed by Mr. Tesla, the field

had 480 polar projections on each side, and from this machine it was

possible to obtain 30,000 alternations per second. As the polar

projections must necessarily be very narrow, very thin wires or sheets

must be used to avoid the eddy current effects. Mr. Tesla has thus

constructed machines with a stationary armature and rotating field, in

which case also the field-coil was supported so that the revolving part

consisted only of a wrought iron body devoid of any wire and also

machines with a rotating armature and stationary field. The machines may

be either drum or disc, but Mr. Tesla's experience shows the latter to

be preferable.

* * * * *

In the course of a very interesting article contributed to the

_Electrical World_ in February, 1891, Mr. Tesla makes some suggestive

remarks on these high frequency machines and his experiences with them,

as well as with other parts of the high frequency apparatus. Part of it

is quoted here and is as follows:--

The writer will incidentally mention that any one who attempts for the

first time to construct such a machine will have a tale of woe to tell.

He will first start out, as a matter of course, by making an armature

with the required number of polar projections. He will then get the

satisfaction of having produced an apparatus which is fit to accompany a

thoroughly Wagnerian opera. It may besides possess the virtue of

converting mechanical energy into heat in a nearly perfect manner. If

there is a reversal in the polarity of the projections, he will get heat

out of the machine; if there is no reversal, the heating will be less,

but the output will be next to nothing. He will then abandon the iron in

the armature, and he will get from the Scylla to the Charybdis. He will

look for one difficulty and will find another, but, after a few trials,

he may get nearly what he wanted.

Among the many experiments which may be performed with such a machine,

of not the least interest are those performed with a high-tension

induction coil. The character of the discharge is completely changed.

The arc is established at much greater distances, and it is so easily

affected by the slightest current of air that it often wriggles around

in the most singular manner. It usually emits the rhythmical sound

peculiar to the alternate current arcs, but the curious point is that

the sound may be heard with a number of alternations far above ten

thousand per second, which by many is considered to be about the limit

of audition. In many respects the coil behaves like a static machine.

Points impair considerably the sparking interval, electricity escaping

from them freely, and from a wire attached to one of the terminals

streams of light issue, as though it were connected to a pole of a

powerful Toepler machine. All these phenomena are, of course, mostly due

to the enormous differences of potential obtained. As a consequence of

the self-induction of the coil and the high frequency, the current is

minute while there is a corresponding rise of pressure. A current

impulse of some strength started in such a coil should persist to flow

no less than four ten-thousandths of a second. As this time is greater

than half the period, it occurs that an opposing electromotive force

begins to act while the current is still flowing. As a consequence, the

pressure rises as in a tube filled with liquid and vibrated rapidly

around its axis. The current is so small that, in the opinion and

involuntary experience of the writer, the discharge of even a very large

coil cannot produce seriously injurious effects, whereas, if the same

coil were operated with a current of lower frequency, though the

electromotive force would be much smaller, the discharge would be most

certainly injurious. This result, however, is due in part to the high

frequency. The writer's experiences tend to show that the higher the

frequency the greater the amount of electrical energy which may be

passed through the body without serious discomfort; whence it seems

certain that human tissues act as condensers.