There is still another and far more striking peculiarity of the brush

discharge produced by very rapidly alternating currents. To observe this

it is best to replace the usual terminals of the coil by two metal

columns insulated with a good thickness of ebonite. It is also well to

close all fissures and cracks with wax so that the brushes cannot form

anywhere except at the tops of the columns. If the conditions are

carefully adjusted--which, of course, must be left to the skill of the

experimenter--so that the potential rises to an enormous value, one may

produce two powerful brushes several inches long, nearly white at their

roots, which in the dark bear a striking resemblance to two flames of a

gas escaping under pressure (Fig. 110). But they do not only _resemble_,

they _are_ veritable flames, for they are hot. Certainly they are not as

hot as a gas burner, _but they would be so if the frequency and the

potential would be sufficiently high_. Produced with, say, twenty

thousand alternations per second, the heat is easily perceptible even if

the potential is not excessively high. The heat developed is, of course,

due to the impact of the air molecules against the terminals and against

each other. As, at the ordinary pressures, the mean free path is

excessively small, it is possible that in spite of the enormous initial

speed imparted to each molecule upon coming in contact with the

terminal, its progress--by collision with other molecules--is retarded

to such an extent, that it does not get away far from the terminal, but

may strike the same many times in succession. The higher the frequency,

the less the molecule is able to get away, and this the more so, as for

a given effect the potential required is smaller; and a frequency is

conceivable--perhaps even obtainable--at which practically the same

molecules would strike the terminal. Under such conditions the exchange

of the molecules would be very slow, and the heat produced at, and very

near, the terminal would be excessive. But if the frequency would go on

increasing constantly, the heat produced would begin to diminish for

obvious reasons. In the positive brush of a static machine the exchange

of the molecules is very rapid, the stream is constantly of one

direction, and there are fewer collisions; hence the heating effect must

be very small. Anything that impairs the facility of exchange tends to

increase the local heat produced. Thus, if a bulb be held over the

terminal of the coil so as to enclose the brush, the air contained in

the bulb is very quickly brought to a high temperature. If a glass tube

be held over the brush so as to allow the draught to carry the brush

upwards, scorching hot air escapes at the top of the tube. Anything held

within the brush is, of course, rapidly heated, and the possibility of

using such heating effects for some purpose or other suggests itself.

When contemplating this singular phenomenon of the hot brush, we cannot

help being convinced that a similar process must take place in the

ordinary flame, and it seems strange that after all these centuries past

of familiarity with the flame, now, in this era of electric lighting and

heating, we are finally led to recognize, that since time immemorial we

have, after all, always had "electric light and heat" at our disposal.

It is also of no little interest to contemplate, that we have a possible

way of producing--by other than chemical means--a veritable flame, which

would give light and heat without any material being consumed, without

any chemical process taking place, and to accomplish this, we only need

to perfect methods of producing enormous frequencies and potentials. I

have no doubt that if the potential could be made to alternate with

sufficient rapidity and power, the brush formed at the end of a wire

would lose its electrical characteristics and would become flamelike.

The flame must be due to electrostatic molecular action.

This phenomenon now explains in a manner which can hardly be doubted the

frequent accidents occurring in storms. It is well known that objects

are often set on fire without the lightning striking them. We shall

presently see how this can happen. On a nail in a roof, for instance, or

on a projection of any kind, more or less conducting, or rendered so by

dampness, a powerful brush may appear. If the lightning strikes

somewhere in the neighborhood the enormous potential may be made to

alternate or fluctuate perhaps many million times a second. The air

molecules are violently attracted and repelled, and by their impact

produce such a powerful heating effect that a fire is started. It is

conceivable that a ship at sea may, in this manner, catch fire at many

points at once. When we consider, that even with the comparatively low

frequencies obtained from a dynamo machine, and with potentials of no

more than one or two hundred thousand volts, the heating effects are

considerable, we may imagine how much more powerful they must be with

frequencies and potentials many times greater; and the above explanation

seems, to say the least, very probable. Similar explanations may have

been suggested, but I am not aware that, up to the present, the heating

effects of a brush produced by a rapidly alternating potential have been

experimentally demonstrated, at least not to such a remarkable degree.

[Illustration: FIG. 111.]

By preventing completely the exchange of the air molecules, the local

heating effect may be so exalted as to bring a body to incandescence.

Thus, for instance, if a small button, or preferably a very thin wire or

filament be enclosed in an unexhausted globe and connected with the

terminal of the coil, it may be rendered incandescent. The phenomenon is

made much more interesting by the rapid spinning round in a circle of

the top of the filament, thus presenting the appearance of a luminous

funnel, Fig. 111, which widens when the potential is increased. When the

potential is small the end of the filament may perform irregular

motions, suddenly changing from one to the other, or it may describe an

ellipse; but when the potential is very high it always spins in a

circle; and so does generally a thin straight wire attached freely to

the terminal of the coil. These motions are, of course, due to the

impact of the molecules, and the irregularity in the distribution of the

potential, owing to the roughness and dissymmetry of the wire or

filament. With a perfectly symmetrical and polished wire such motions

would probably not occur. That the motion is not likely to be due to

others causes is evident from the fact that it is not of a definite

direction, and that in a very highly exhausted globe it ceases

altogether. The possibility of bringing a body to incandescence in an

exhausted globe, or even when not at all enclosed, would seem to afford

a possible way of obtaining light effects, which, in perfecting methods

of producing rapidly alternating potentials, might be rendered available

for useful purposes.

[Illustration: FIG. 112a.]

In employing a commercial coil, the production of very powerful brush

effects is attended with considerable difficulties, for when these high

frequencies and enormous potentials are used, the best insulation is apt

to give way. Usually the coil is insulated well enough to stand the

strain from convolution to convolution, since two double silk covered

paraffined wires will withstand a pressure of several thousand volts;

the difficulty lies principally in preventing the breaking through from

the secondary to the primary, which is greatly facilitated by the

streams issuing from the latter. In the coil, of course, the strain is

greatest from section to section, but usually in a larger coil there are

so many sections that the danger of a sudden giving way is not very

great. No difficulty will generally be encountered in that direction,

and besides, the liability of injuring the coil internally is very much

reduced by the fact that the effect most likely to be produced is simply

a gradual heating, which, when far enough advanced, could not fail to

be observed. The principal necessity is then to prevent the streams

between the primary and the tube, not only on account of the heating and

possible injury, but also because the streams may diminish very

considerably the potential difference available at the terminals. A few

hints as to how this may be accomplished will probably be found useful

in most of these experiments with the ordinary induction coil.

[Illustration: FIG. 112b.]

One of the ways is to wind a short primary, Fig. 112a, so that the

difference of potential is not at that length great enough to cause the

breaking forth of the streams through the insulating tube. The length of

the primary should be determined by experiment. Both the ends of the

coil should be brought out on one end through a plug of insulating

material fitting in the tube as illustrated. In such a disposition one

terminal of the secondary is attached to a body, the surface of which is

determined with the greatest care so as to produce the greatest rise in

the potential. At the other terminal a powerful brush appears, which may

be experimented upon.

The above plan necessitates the employment of a primary of comparatively

small size, and it is apt to heat when powerful effects are desirable

for a certain length of time. In such a case it is better to employ a

larger coil, Fig. 112b, and introduce it from one side of the tube,

until the streams begin to appear. In this case the nearest terminal of

the secondary may be connected to the primary or to the ground, which is

practically the same thing, if the primary is connected directly to the

machine. In the case of ground connections it is well to determine

experimentally the frequency which is best suited under the conditions

of the test. Another way of obviating the streams, more or less, is to

make the primary in sections and supply it from separate, well insulated

sources.

In many of these experiments, when powerful effects are wanted for a

short time, it is advantageous to use iron cores with the primaries. In

such case a very large primary coil may be wound and placed side by side

with the secondary, and, the nearest terminal of the latter being

connected to the primary, a laminated iron core is introduced through

the primary into the secondary as far as the streams will permit. Under

these conditions an excessively powerful brush, several inches long,

which may be appropriately called "St. Elmo's hot fire," may be caused

to appear at the other terminal of the secondary, producing striking

effects. It is a most powerful ozonizer, so powerful indeed, that only a

few minutes are sufficient to fill the whole room with the smell of

ozone, and it undoubtedly possesses the quality of exciting chemical

affinities.

For the production of ozone, alternating currents of very high frequency

are eminently suited, not only on account of the advantages they offer

in the way of conversion but also because of the fact, that the

ozonizing action of a discharge is dependent on the frequency as well as

on the potential, this being undoubtedly confirmed by observation.

In these experiments if an iron core is used it should be carefully

watched, as it is apt to get excessively hot in an incredibly short

time. To give an idea of the rapidity of the heating, I will state, that

by passing a powerful current through a coil with many turns, the

inserting within the same of a thin iron wire for no more than one

second's time is sufficient to heat the wire to something like 100 deg. C.

But this rapid heating need not discourage us in the use of iron cores

in connection with rapidly alternating currents. I have for a long time

been convinced that in the industrial distribution by means of

transformers, some such plan as the following might be practicable. We

may use a comparatively small iron core, subdivided, or perhaps not even

subdivided. We may surround this core with a considerable thickness of

material which is fire-proof and conducts the heat poorly, and on top of

that we may place the primary and secondary windings. By using either

higher frequencies or greater magnetizing forces, we may by hysteresis

and eddy currents heat the iron core so far as to bring it nearly to its

maximum permeability, which, as Hopkinson has shown, may be as much as

sixteen times greater than that at ordinary temperatures. If the iron

core were perfectly enclosed, it would not be deteriorated by the heat,

and, if the enclosure of fire-proof material would be sufficiently

thick, only a limited amount of energy could be radiated in spite of the

high temperature. Transformers have been constructed by me on that plan,

but for lack of time, no thorough tests have as yet been made.

Another way of adapting the iron core to rapid alternations, or,

generally speaking, reducing the frictional losses, is to produce by

continuous magnetization a flow of something like seven thousand or

eight thousand lines per square centimetre through the core, and then

work with weak magnetizing forces and preferably high frequencies around

the point of greatest permeability. A higher efficiency of conversion

and greater output are obtainable in this manner. I have also employed

this principle in connection with machines in which there is no reversal

of polarity. In these types of machines, as long as there are only few

pole projections, there is no great gain, as the maxima and minima of

magnetization are far from the point of maximum permeability; but when

the number of the pole projections is very great, the required rate of

change may be obtained, without the magnetization varying so far as to

depart greatly from the point of maximum permeability, and the gain is

considerable.

The above described arrangements refer only to the use of commercial

coils as ordinarily constructed. If it is desired to construct a coil

for the express purpose of performing with it such experiments as I have

described, or, generally, rendering it capable of withstanding the

greatest possible difference of potential, then a construction as

indicated in Fig. 113 will be found of advantage. The coil in this case

is formed of two independent parts which are wound oppositely, the

connection between both being made near the primary. The potential in

the middle being zero, there is not much tendency to jump to the primary

and not much insulation is required. In some cases the middle point may,

however, be connected to the primary or to the ground. In such a coil

the places of greatest difference of potential are far apart and the

coil is capable of withstanding an enormous strain. The two parts may be

movable so as to allow a slight adjustment of the capacity effect.

As to the manner of insulating the coil, it will be found convenient to

proceed in the following way: First, the wire should be boiled in

paraffine until all the air is out; then the coil is wound by running

the wire through melted paraffine, merely for the purpose of fixing the

wire. The coil is then taken off from the spool, immersed in a

cylindrical vessel filled with pure melted wax and boiled for a long

time until the bubbles cease to appear. The whole is then left to cool

down thoroughly, and then the mass is taken out of the vessel and turned

up in a lathe. A coil made in this manner and with care is capable of

withstanding enormous potential differences.

[Illustration: FIG. 113.]

It may be found convenient to immerse the coil in paraffine oil or some

other kind of oil; it is a most effective way of insulating, principally

on account of the perfect exclusion of air, but it may be found that,

after all, a vessel filled with oil is not a very convenient thing to

handle in a laboratory.

If an ordinary coil can be dismounted, the primary may be taken out of

the tube and the latter plugged up at one end, filled with oil, and the

primary reinserted. This affords an excellent insulation and prevents

the formation of the streams.

Of all the experiments which may be performed with rapidly alternating

currents the most interesting are those which concern the production of

a practical illuminant. It cannot be denied that the present methods,

though they were brilliant advances, are very wasteful. Some better

methods must be invented, some more perfect apparatus devised. Modern

research has opened new possibilities for the production of an efficient

source of light, and the attention of all has been turned in the

direction indicated by able pioneers. Many have been carried away by

the enthusiasm and passion to discover, but in their zeal to reach

results, some have been misled. Starting with the idea of producing

electro-magnetic waves, they turned their attention, perhaps, too much

to the study of electro-magnetic effects, and neglected the study of

electrostatic phenomena. Naturally, nearly every investigator availed

himself of an apparatus similar to that used in earlier experiments. But

in those forms of apparatus, while the electro-magnetic inductive

effects are enormous, the electrostatic effects are excessively small.

In the Hertz experiments, for instance, a high tension induction coil is

short circuited by an arc, the resistance of which is very small, the

smaller, the more capacity is attached to the terminals; and the

difference of potential at these is enormously diminished. On the other

hand, when the discharge is not passing between the terminals, the

static effects may be considerable, but only qualitatively so, not

quantitatively, since their rise and fall is very sudden, and since

their frequency is small. In neither case, therefore, are powerful

electrostatic effects perceivable. Similar conditions exist when, as in

some interesting experiments of Dr. Lodge, Leyden jars are discharged

disruptively. It has been thought--and I believe asserted--that in such

cases most of the energy is radiated into space. In the light of the

experiments which I have described above, it will now not be thought so.

I feel safe in asserting that in such cases most of the energy is partly

taken up and converted into heat in the arc of the discharge and in the

conducting and insulating material of the jar, some energy being, of

course, given off by electrification of the air; but the amount of the

directly radiated energy is very small.

When a high tension induction coil, operated by currents alternating

only 20,000 times a second, has its terminals closed through even a very

small jar, practically all the energy passes through the dielectric of

the jar, which is heated, and the electrostatic effects manifest

themselves outwardly only to a very weak degree. Now the external

circuit of a Leyden jar, that is, the arc and the connections of the

coatings, may be looked upon as a circuit generating alternating

currents of excessively high frequency and fairly high potential, which

is closed through the coatings and the dielectric between them, and from

the above it is evident that the external electrostatic effects must be

very small, even if a recoil circuit be used. These conditions make it

appear that with the apparatus usually at hand, the observation of

powerful electrostatic effects was impossible, and what experience has

been gained in that direction is only due to the great ability of the

investigators.

But powerful electrostatic effects are a _sine qua non_ of light

production on the lines indicated by theory. Electro-magnetic effects

are primarily unavailable, for the reason that to produce the required

effects we would have to pass current impulses through a conductor,

which, long before the required frequency of the impulses could be

reached, would cease to transmit them. On the other hand,

electro-magnetic waves many times longer than those of light, and

producible by sudden discharge of a condenser, could not be utilized, it

would seem, except we avail ourselves of their effect upon conductors as

in the present methods, which are wasteful. We could not affect by means

of such waves the static molecular or atomic charges of a gas, cause

them to vibrate and to emit light. Long transverse waves cannot,

apparently, produce such effects, since excessively small

electro-magnetic disturbances may pass readily through miles of air.

Such dark waves, unless they are of the length of true light waves,

cannot, it would seem, excite luminous radiation in a Geissler tube, and

the luminous effects, which are producible by induction in a tube devoid

of electrodes, I am inclined to consider as being of an electrostatic

nature.

To produce such luminous effects, straight electrostatic thrusts are

required; these, whatever be their frequency, may disturb the molecular

charges and produce light. Since current impulses of the required

frequency cannot pass through a conductor of measurable dimensions, we

must work with a gas, and then the production of powerful electrostatic

effects becomes an imperative necessity.

It has occurred to me, however, that electrostatic effects are in many

ways available for the production of light. For instance, we may place a

body of some refractory material in a closed, and preferably more or

less exhausted, globe, connect it to a source of high, rapidly

alternating potential, causing the molecules of the gas to strike it

many times a second at enormous speeds, and in this manner, with

trillions of invisible hammers, pound it until it gets incandescent; or

we may place a body in a very highly exhausted globe, in a non-striking

vacuum, and, by employing very high frequencies and potentials,

transfer sufficient energy from it to other bodies in the vicinity, or

in general to the surroundings, to maintain it at any degree of

incandescence; or we may, by means of such rapidly alternating high

potentials, disturb the ether carried by the molecules of a gas or their

static charges, causing them to vibrate and to emit light.

But, electrostatic effects being dependent upon the potential and

frequency, to produce the most powerful action it is desirable to

increase both as far as practicable. It may be possible to obtain quite

fair results by keeping either of these factors small, provided the

other is sufficiently great; but we are limited in both directions. My

experience demonstrates that we cannot go below a certain frequency,

for, first, the potential then becomes so great that it is dangerous;

and, secondly, the light production is less efficient.

I have found that, by using the ordinary low frequencies, the

physiological effect of the current required to maintain at a certain

degree of brightness a tube four feet long, provided at the ends with

outside and inside condenser coatings, is so powerful that, I think, it

might produce serious injury to those not accustomed to such shocks;

whereas, with twenty thousand alternations per second, the tube may be

maintained at the same degree of brightness without any effect being

felt. This is due principally to the fact that a much smaller potential

is required to produce the same light effect, and also to the higher

efficiency in the light production. It is evident that the efficiency in

such cases is the greater, the higher the frequency, for the quicker the

process of charging and discharging the molecules, the less energy will

be lost in the form of dark radiation. But, unfortunately, we cannot go

beyond a certain frequency on account of the difficulty of producing and

conveying the effects.

I have stated above that a body inclosed in an unexhausted bulb may be

intensely heated by simply connecting it with a source of rapidly

alternating potential. The heating in such a case is, in all

probability, due mostly to the bombardment of the molecules of the gas

contained in the bulb. When the bulb is exhausted, the heating of the

body is much more rapid, and there is no difficulty whatever in bringing

a wire or filament to any degree of incandescence by simply connecting

it to one terminal of a coil of the proper dimensions. Thus, if the

well-known apparatus of Prof. Crookes, consisting of a bent platinum

wire with vanes mounted over it (Fig. 114), be connected to one

terminal of the coil--either one or both ends of the platinum wire being

connected--the wire is rendered almost instantly incandescent, and the

mica vanes are rotated as though a current from a battery were used. A

thin carbon filament, or, preferably, a button of some refractory

material (Fig. 115), even if it be a comparatively poor conductor,

inclosed in an exhausted globe, may be rendered highly incandescent; and

in this manner a simple lamp capable of giving any desired candle power

is provided.

The success of lamps of this kind would depend largely on the selection

of the light-giving bodies contained within the bulb. Since, under the

conditions described, refractory bodies--which are very poor conductors

and capable of withstanding for a long time excessively high degrees of

temperature--may be used, such illuminating devices may be rendered

successful.

[Illustration: FIG. 114.]

[Illustration: FIG. 115.]

It might be thought at first that if the bulb, containing the filament

or button of refractory material, be perfectly well exhausted--that is,

as far as it can be done by the use of the best apparatus--the heating

would be much less intense, and that in a perfect vacuum it could not

occur at all. This is not confirmed by my experience; quite the

contrary, the better the vacuum the more easily the bodies are brought

to incandescence. This result is interesting for many reasons.

At the outset of this work the idea presented itself to me, whether two

bodies of refractory material enclosed in a bulb exhausted to such a

degree that the discharge of a large induction coil, operated in the

usual manner, cannot pass through, could be rendered incandescent by

mere condenser action. Obviously, to reach this result enormous

potential differences and very high frequencies are required, as is

evident from a simple calculation.

But such a lamp would possess a vast advantage over an ordinary

incandescent lamp in regard to efficiency. It is well-known that the

efficiency of a lamp is to some extent a function of the degree of

incandescence, and that, could we but work a filament at many times

higher degrees of incandescence, the efficiency would be much greater.

In an ordinary lamp this is impracticable on account of the destruction

of the filament, and it has been determined by experience how far it is

advisable to push the incandescence. It is impossible to tell how much

higher efficiency could be obtained if the filament could withstand

indefinitely, as the investigation to this end obviously cannot be

carried beyond a certain stage; but there are reasons for believing that

it would be very considerably higher. An improvement might be made in

the ordinary lamp by employing a short and thick carbon; but then the

leading-in wires would have to be thick, and, besides, there are many

other considerations which render such a modification entirely

impracticable. But in a lamp as above described, the leading in wires

may be very small, the incandescent refractory material may be in the

shape of blocks offering a very small radiating surface, so that less

energy would be required to keep them at the desired incandescence; and

in addition to this, the refractory material need not be carbon, but may

be manufactured from mixtures of oxides, for instance, with carbon or

other material, or may be selected from bodies which are practically

non-conductors, and capable of withstanding enormous degrees of

temperature.

All this would point to the possibility of obtaining a much higher

efficiency with such a lamp than is obtainable in ordinary lamps. In my

experience it has been demonstrated that the blocks are brought to high

degrees of incandescence with much lower potentials than those

determined by calculation, and the blocks may be set at greater

distances from each other. We may freely assume, and it is probable,

that the molecular bombardment is an important element in the heating,

even if the globe be exhausted with the utmost care, as I have done; for

although the number of the molecules is, comparatively speaking,

insignificant, yet on account of the mean free path being very great,

there are fewer collisions, and the molecules may reach much higher

speeds, so that the heating effect due to this cause may be

considerable, as in the Crookes experiments with radiant matter.

But it is likewise possible that we have to deal here with an increased

facility of losing the charge in very high vacuum, when the potential is

rapidly alternating, in which case most of the heating would be directly

due to the surging of the charges in the heated bodies. Or else the

observed fact may be largely attributable to the effect of the points

which I have mentioned above, in consequence of which the blocks or

filaments contained in the vacuum are equivalent to condensers of many

times greater surface than that calculated from their geometrical

dimensions. Scientific men still differ in opinion as to whether a

charge should, or should not, be lost in a perfect vacuum, or in other

words, whether ether is, or is not, a conductor. If the former were the

case, then a thin filament enclosed in a perfectly exhausted globe, and

connected to a source of enormous, steady potential, would be brought to

incandescence.

[Illustration: FIG. 116.]

[Illustration: FIG. 117.]

Various forms of lamps on the above described principle, with the

refractory bodies in the form of filaments, Fig. 116, or blocks, Fig.

117, have been constructed and operated by me, and investigations are

being carried on in this line. There is no difficulty in reaching such

high degrees of incandescence that ordinary carbon is to all appearance

melted and volatilized. If the vacuum could be made absolutely perfect,

such a lamp, although inoperative with apparatus ordinarily used, would,

if operated with currents of the required character, afford an

illuminant which would never be destroyed, and which would be far more

efficient than an ordinary incandescent lamp. This perfection can, of

course, never be reached, and a very slow destruction and gradual

diminution in size always occurs, as in incandescent filaments; but

there is no possibility of a sudden and premature disabling which occurs

in the latter by the breaking of the filament, especially when the

incandescent bodies are in the shape of blocks.

With these rapidly alternating potentials there is, however, no

necessity of enclosing two blocks in a globe, but a single block, as in

Fig. 115, or filament, Fig. 118, may be used. The potential in this case

must of course be higher, but is easily obtainable, and besides it is

not necessarily dangerous.

[Illustration: FIG. 118.]

The facility with which the button or filament in such a lamp is brought

to incandescence, other things being equal, depends on the size of the

globe. If a perfect vacuum could be obtained, the size of the globe

would not be of importance, for then the heating would be wholly due to

the surging of the charges, and all the energy would be given off to the

surroundings by radiation. But this can never occur in practice. There

is always some gas left in the globe, and although the exhaustion may be

carried to the highest degree, still the space inside of the bulb must

be considered as conducting when such high potentials are used, and I

assume that, in estimating the energy that may be given off from the

filament to the surroundings, we may consider the inside surface of the

bulb as one coating of a condenser, the air and other objects

surrounding the bulb forming the other coating. When the alternations

are very low there is no doubt that a considerable portion of the energy

is given off by the electrification of the surrounding air.

In order to study this subject better, I carried on some experiments

with excessively high potentials and low frequencies. I then observed

that when the hand is approached to the bulb,--the filament being

connected with one terminal of the coil,--a powerful vibration is felt,

being due to the attraction and repulsion of the molecules of the air

which are electrified by induction through the glass. In some cases when

the action is very intense I have been able to hear a sound, which must

be due to the same cause.

[Illustration: FIG. 119.]

[Illustration: FIG. 120.]

When the alternations are low, one is apt to get an excessively powerful

shock from the bulb. In general, when one attaches bulbs or objects of

some size to the terminals of the coil, one should look out for the rise

of potential, for it may happen that by merely connecting a bulb or

plate to the terminal, the potential may rise to many times its original

value. When lamps are attached to the terminals, as illustrated in Fig.

119, then the capacity of the bulbs should be such as to give the

maximum rise of potential under the existing conditions. In this manner

one may obtain the required potential with fewer turns of wire.

The life of such lamps as described above depends, of course, largely on

the degree of exhaustion, but to some extent also on the shape of the

block of refractory material. Theoretically it would seem that a small

sphere of carbon enclosed in a sphere of glass would not suffer

deterioration from molecular bombardment, for, the matter in the globe

being radiant, the molecules would move in straight lines, and would

seldom strike the sphere obliquely. An interesting thought in connection

with such a lamp is, that in it "electricity" and electrical energy

apparently must move in the same lines.

[Illustration: FIG. 121a.]

[Illustration: FIG. 121b.]

The use of alternating currents of very high frequency makes it possible

to transfer, by electrostatic or electromagnetic induction through the

glass of a lamp, sufficient energy to keep a filament at incandescence

and so do away with the leading-in wires. Such lamps have been proposed,

but for want of proper apparatus they have not been successfully

operated. Many forms of lamps on this principle with continuous and

broken filaments have been constructed by me and experimented upon. When

using a secondary enclosed within the lamp, a condenser is

advantageously combined with the secondary. When the transference is

effected by electrostatic induction, the potentials used are, of course,

very high with frequencies obtainable from a machine. For instance, with

a condenser surface of forty square centimetres, which is not

impracticably large, and with glass of good quality 1 mm. thick, using

currents alternating twenty thousand times a second, the potential

required is approximately 9,000 volts. This may seem large, but since

each lamp may be included in the secondary of a transformer of very

small dimensions, it would not be inconvenient, and, moreover, it would

not produce fatal injury. The transformers would all be preferably in

series. The regulation would offer no difficulties, as with currents of

such frequencies it is very easy to maintain a constant current.

In the accompanying engravings some of the types of lamps of this kind

are shown. Fig. 120 is such a lamp with a broken filament, and Figs. 121

A and 121 B one with a single outside and inside coating and a single

filament. I have also made lamps with two outside and inside coatings

and a continuous loop connecting the latter. Such lamps have been

operated by me with current impulses of the enormous frequencies

obtainable by the disruptive discharge of condensers.

The disruptive discharge of a condenser is especially suited for

operating such lamps--with no outward electrical connections--by means

of electromagnetic induction, the electromagnetic inductive effects

being excessively high; and I have been able to produce the desired

incandescence with only a few short turns of wire. Incandescence may

also be produced in this manner in a simple closed filament.

Leaving now out of consideration the practicability of such lamps, I

would only say that they possess a beautiful and desirable feature,

namely, that they can be rendered, at will, more or less brilliant

simply by altering the relative position of the outside and inside

condenser coatings, or inducing and induced circuits.

When a lamp is lighted by connecting it to one terminal only of the

source, this may be facilitated by providing the globe with an outside

condenser coating, which serves at the same time as a reflector, and

connecting this to an insulated body of some size. Lamps of this kind

are illustrated in Fig. 122 and Fig. 123. Fig. 124 shows the plan of

connection. The brilliancy of the lamp may, in this case, be regulated

within wide limits by varying the size of the insulated metal plate to

which the coating is connected.

It is likewise practicable to light with one leading wire lamps such as

illustrated in Fig. 116 and Fig. 117, by connecting one terminal of the

lamp to one terminal of the source, and the other to an insulated body

of the required size. In all cases the insulated body serves to give off

the energy into the surrounding space, and is equivalent to a return

wire. Obviously, in the two last-named cases, instead of connecting the

wires to an insulated body, connections may be made to the ground.

The experiments which will prove most suggestive and of most interest to

the investigator are probably those performed with exhausted tubes. As

might be anticipated, a source of such rapidly alternating potentials is

capable of exciting the tubes at a considerable distance, and the light

effects produced are remarkable.

[Illustration: FIG. 122.]

[Illustration: FIG. 123.]

During my investigations in this line I endeavored to excite tubes,

devoid of any electrodes, by electromagnetic induction, making the tube

the secondary of the induction device, and passing through the primary

the discharges of a Leyden jar. These tubes were made of many shapes,

and I was able to obtain luminous effects which I then thought were due

wholly to electromagnetic induction. But on carefully investigating the

phenomena I found that the effects produced were more of an

electrostatic nature. It may be attributed to this circumstance that

this mode of exciting tubes is very wasteful, namely, the primary

circuit being closed, the potential, and consequently the electrostatic

inductive effect, is much diminished.

When an induction coil, operated as above described, is used, there is

no doubt that the tubes are excited by electrostatic induction, and that

electromagnetic induction has little, if anything, to do with the

phenomena.

[Illustration: FIG. 124.]

This is evident from many experiments. For instance, if a tube be taken

in one hand, the observer being near the coil, it is brilliantly lighted

and remains so no matter in what position it is held relatively to the

observer's body. Were the action electromagnetic, the tube could not be

lighted when the observer's body is interposed between it and the coil,

or at least its luminosity should be considerably diminished. When the

tube is held exactly over the centre of the coil--the latter being wound

in sections and the primary placed symmetrically to the secondary--it

may remain completely dark, whereas it is rendered intensely luminous by

moving it slightly to the right or left from the centre of the coil. It

does not light because in the middle both halves of the coil neutralize

each other, and the electric potential is zero. If the action were

electromagnetic, the tube should light best in the plane through the

centre of the coil, since the electromagnetic effect there should be a

maximum. When an arc is established between the terminals, the tubes and

lamps in the vicinity of the coil go out, but light up again when the

arc is broken, on account of the rise of potential. Yet the

electromagnetic effect should be practically the same in both cases.

By placing a tube at some distance from the coil, and nearer to one

terminal--preferably at a point on the axis of the coil--one may light

it by touching the remote terminal with an insulated body of some size

or with the hand, thereby raising the potential at that terminal nearer

to the tube. If the tube is shifted nearer to the coil so that it is

lighted by the action of the nearer terminal, it may be made to go out

by holding, on an insulated support, the end of a wire connected to the

remote terminal, in the vicinity of the nearer terminal, by this means

counteracting the action of the latter upon the tube. These effects are

evidently electrostatic. Likewise, when a tube is placed at a

considerable distance from the coil, the observer may, standing upon an

insulated support between coil and tube, light the latter by approaching

the hand to it; or he may even render it luminous by simply stepping

between it and the coil. This would be impossible with electro-magnetic

induction, for the body of the observer would act as a screen.

When the coil is energized by excessively weak currents, the

experimenter may, by touching one terminal of the coil with the tube,

extinguish the latter, and may again light it by bringing it out of

contact with the terminal and allowing a small arc to form. This is

clearly due to the respective lowering and raising of the potential at

that terminal. In the above experiment, when the tube is lighted through

a small arc, it may go out when the arc is broken, because the

electrostatic inductive effect alone is too weak, though the potential

may be much higher; but when the arc is established, the electrification

of the end of the tube is much greater, and it consequently lights.

If a tube is lighted by holding it near to the coil, and in the hand

which is remote, by grasping the tube anywhere with the other hand, the

part between the hands is rendered dark, and the singular effect of

wiping out the light of the tube may be produced by passing the hand

quickly along the tube and at the same time withdrawing it gently from

the coil, judging properly the distance so that the tube remains dark

afterwards.

If the primary coil is placed sidewise, as in Fig. 112 B for instance,

and an exhausted tube be introduced from the other side in the hollow

space, the tube is lighted most intensely because of the increased

condenser action, and in this position the striae are most sharply

defined. In all these experiments described, and in many others, the

action is clearly electrostatic.

The effects of screening also indicate the electrostatic nature of the

phenomena and show something of the nature of electrification through

the air. For instance, if a tube is placed in the direction of the axis

of the coil, and an insulated metal plate be interposed, the tube will

generally increase in brilliancy, or if it be too far from the coil to

light, it may even be rendered luminous by interposing an insulated

metal plate. The magnitude of the effects depends to some extent on the

size of the plate. But if the metal plate be connected by a wire to the

ground, its interposition will always make the tube go out even if it be

very near the coil. In general, the interposition of a body between the

coil and tube, increases or diminishes the brilliancy of the tube, or

its facility to light up, according to whether it increases or

diminishes the electrification. When experimenting with an insulated

plate, the plate should not be taken too large, else it will generally

produce a weakening effect by reason of its great facility for giving

off energy to the surroundings.

If a tube be lighted at some distance from the coil, and a plate of hard

rubber or other insulating substance be interposed, the tube may be made

to go out. The interposition of the dielectric in this case only

slightly increases the inductive effect, but diminishes considerably the

electrification through the air.

In all cases, then, when we excite luminosity in exhausted tubes by

means of such a coil, the effect is due to the rapidly alternating

electrostatic potential; and, furthermore, it must be attributed to the

harmonic alternation produced directly by the machine, and not to any

superimposed vibration which might be thought to exist. Such

superimposed vibrations are impossible when we work with an alternate

current machine. If a spring be gradually tightened and released, it

does not perform independent vibrations; for this a sudden release is

necessary. So with the alternate currents from a dynamo machine; the

medium is harmonically strained and released, this giving rise to only

one kind of waves; a sudden contact or break, or a sudden giving way of

the dielectric, as in the disruptive discharge of a Leyden jar, are

essential for the production of superimposed waves.

In all the last described experiments, tubes devoid of any electrodes

may be used, and there is no difficulty in producing by their means

sufficient light to read by. The light effect is, however, considerably

increased by the use of phosphorescent bodies such as yttria, uranium

glass, etc. A difficulty will be found when the phosphorescent material

is used, for with these powerful effects, it is carried gradually away,

and it is preferable to use material in the form of a solid.

Instead of depending on induction at a distance to light the tube, the

same may be provided with an external--and, if desired, also with an

internal--condenser coating, and it may then be suspended anywhere in

the room from a conductor connected to one terminal of the coil, and in

this manner a soft illumination may be provided.

[Illustration: FIG. 125.]

The ideal way of lighting a hall or room would, however, be to produce

such a condition in it that an illuminating device could be moved and

put anywhere, and that it is lighted, no matter where it is put and

without being electrically connected to anything. I have been able to

produce such a condition by creating in the room a powerful, rapidly

alternating electrostatic field. For this purpose I suspend a sheet of

metal a distance from the ceiling on insulating cords and connect it to

one terminal of the induction coil, the other terminal being preferably

connected to the ground. Or else I suspend two sheets as illustrated in

Fig. 125, each sheet being connected with one of the terminals of the

coil, and their size being carefully determined. An exhausted tube may

then be carried in the hand anywhere between the sheets or placed

anywhere, even a certain distance beyond them; it remains always

luminous.

In such an electrostatic field interesting phenomena may be observed,

especially if the alternations are kept low and the potentials

excessively high. In addition to the luminous phenomena mentioned, one

may observe that any insulated conductor gives sparks when the hand or

another object is approached to it, and the sparks may often be

powerful. When a large conducting object is fastened on an insulating

support, and the hand approached to it, a vibration, due to the

rythmical motion of the air molecules is felt, and luminous streams may

be perceived when the hand is held near a pointed projection. When a

telephone receiver is made to touch with one or both of its terminals an

insulated conductor of some size, the telephone emits a loud sound; it

also emits a sound when a length of wire is attached to one or both

terminals, and with very powerful fields a sound may be perceived even

without any wire.

How far this principle is capable of practical application, the future

will tell. It might be thought that electrostatic effects are unsuited

for such action at a distance. Electromagnetic inductive effects, if

available for the production of light, might be thought better suited.

It is true the electrostatic effects diminish nearly with the cube of

the distance from the coil, whereas the electromagnetic inductive

effects diminish simply with the distance. But when we establish an

electrostatic field of force, the condition is very different, for then,

instead of the differential effect of both the terminals, we get their

conjoint effect. Besides, I would call attention to the effect, that in

an alternating electrostatic field, a conductor, such as an exhausted

tube, for instance, tends to take up most of the energy, whereas in an

electromagnetic alternating field the conductor tends to take up the

least energy, the waves being reflected with but little loss. This is

one reason why it is difficult to excite an exhausted tube, at a

distance, by electromagnetic induction. I have wound coils of very large

diameter and of many turns of wire, and connected a Geissler tube to the

ends of the coil with the object of exciting the tube at a distance; but

even with the powerful inductive effects producible by Leyden jar

discharges, the tube could not be excited unless at a very small

distance, although some judgment was used as to the dimensions of the

coil. I have also found that even the most powerful Leyden jar

discharges are capable of exciting only feeble luminous effects in a

closed exhausted tube, and even these effects upon thorough examination

I have been forced to consider of an electrostatic nature.

How then can we hope to produce the required effects at a distance by

means of electromagnetic action, when even in the closest proximity to

the source of disturbance, under the most advantageous conditions, we

can excite but faint luminosity? It is true that when acting at a

distance we have the resonance to help us out. We can connect an

exhausted tube, or whatever the illuminating device may be, with an

insulated system of the proper capacity, and so it may be possible to

increase the effect qualitatively, and only qualitatively, for we would

not get _more_ energy through the device. So we may, by resonance

effect, obtain the required electromotive force in an exhausted tube,

and excite faint luminous effects, but we cannot get enough energy to

render the light practically available, and a simple calculation, based

on experimental results, shows that even if all the energy which a tube

would receive at a certain distance from the source should be wholly

converted into light, it would hardly satisfy the practical

requirements. Hence the necessity of directing, by means of a conducting

circuit, the energy to the place of transformation. But in so doing we

cannot very sensibly depart from present methods, and all we could do

would be to improve the apparatus.

From these considerations it would seem that if this ideal way of

lighting is to be rendered practicable it will be only by the use of

electrostatic effects. In such a case the most powerful electrostatic

inductive effects are needed; the apparatus employed must, therefore, be

capable of producing high electrostatic potentials changing in value

with extreme rapidity. High frequencies are especially wanted, for

practical considerations make it desirable to keep down the potential.

By the employment of machines, or, generally speaking, of any

mechanical apparatus, but low frequencies can be reached; recourse must,

therefore, be had to some other means. The discharge of a condenser

affords us a means of obtaining frequencies by far higher than are

obtainable mechanically, and I have accordingly employed condensers in

the experiments to the above end.

When the terminals of a high tension induction coil, Fig. 126, are

connected to a Leyden jar, and the latter is discharging disruptively

into a circuit, we may look upon the arc playing between the knobs as

being a source of alternating, or generally speaking, undulating

currents, and then we have to deal with the familiar system of a

generator of such currents, a circuit connected to it, and a condenser

bridging the circuit. The condenser in such case is a veritable

transformer, and since the frequency is excessive, almost any ratio in

the strength of the currents in both the branches may be obtained. In

reality the analogy is not quite complete, for in the disruptive

discharge we have most generally a fundamental instantaneous variation

of comparatively low frequency, and a superimposed harmonic vibration,

and the laws governing the flow of currents are not the same for both.

In converting in this manner, the ratio of conversion should not be too

great, for the loss in the arc between the knobs increases with the

square of the current, and if the jar be discharged through very thick

and short conductors, with the view of obtaining a very rapid

oscillation, a very considerable portion of the energy stored is lost.

On the other hand, too small ratios are not practicable for many obvious

reasons.

As the converted currents flow in a practically closed circuit, the

electrostatic effects are necessarily small, and I therefore convert

them into currents or effects of the required character. I have effected

such conversions in several ways. The preferred plan of connections is

illustrated in Fig. 127. The manner of operating renders it easy to

obtain by means of a small and inexpensive apparatus enormous

differences of potential which have been usually obtained by means of

large and expensive coils. For this it is only necessary to take an

ordinary small coil, adjust to it a condenser and discharging circuit,

forming the primary of an auxiliary small coil, and convert upward. As

the inductive effect of the primary currents is excessively great, the

second coil need have comparatively but very few turns. By properly

adjusting the elements, remarkable results may be secured.

In endeavoring to obtain the required electrostatic effects in this

manner, I have, as might be expected, encountered many difficulties

which I have been gradually overcoming, but I am not as yet prepared to

dwell upon my experiences in this direction.

I believe that the disruptive discharge of a condenser will play an

important part in the future, for it offers vast possibilities, not only

in the way of producing light in a more efficient manner and in the line

indicated by theory, but also in many other respects.

[Illustration: FIG. 126.]

For years the efforts of inventors have been directed towards obtaining

electrical energy from heat by means of the thermopile. It might seem

invidious to remark that but few know what is the real trouble with the

thermopile. It is not the inefficiency or small output--though these are

great drawbacks--but the fact that the thermopile has its phylloxera,

that is, that by constant use it is deteriorated, which has thus far

prevented its introduction on an industrial scale. Now that all modern

research seems to point with certainty to the use of electricity of

excessively high tension, the question must present itself to many

whether it is not possible to obtain in a practicable manner this form

of energy from heat. We have been used to look upon an electrostatic

machine as a plaything, and somehow we couple with it the idea of the

inefficient and impractical. But now we must think differently, for now

we know that everywhere we have to deal with the same forces, and that

it is a mere question of inventing proper methods or apparatus for

rendering them available.

In the present systems of electrical distribution, the employment of the

iron with its wonderful magnetic properties allows us to reduce

considerably the size of the apparatus; but, in spite of this, it is

still very cumbersome. The more we progress in the study of electric and

magnetic phenomena, the more we become convinced that the present

methods will be short-lived. For the production of light, at least, such

heavy machinery would seem to be unnecessary. The energy required is

very small, and if light can be obtained as efficiently as,

theoretically, it appears possible, the apparatus need have but a very

small output. There being a strong probability that the illuminating

methods of the future will involve the use of very high potentials, it

seems very desirable to perfect a contrivance capable of converting the

energy of heat into energy of the requisite form. Nothing to speak of

has been done towards this end, for the thought that electricity of some

50,000 or 100,000 volts pressure or more, even if obtained, would be

unavailable for practical purposes, has deterred inventors from working

in this direction.

[Illustration: FIG. 127.]

In Fig. 126 a plan of connections is shown for converting currents of

high, into currents of low, tension by means of the disruptive discharge

of a condenser. This plan has been used by me frequently for operating a

few incandescent lamps required in the laboratory. Some difficulties

have been encountered in the arc of the discharge which I have been able

to overcome to a great extent; besides this, and the adjustment

necessary for the proper working, no other difficulties have been met

with, and it was easy to operate ordinary lamps, and even motors, in

this manner. The line being connected to the ground, all the wires could

be handled with perfect impunity, no matter how high the potential at

the terminals of the condenser. In these experiments a high tension

induction coil, operated from a battery or from an alternate current

machine, was employed to charge the condenser; but the induction coil

might be replaced by an apparatus of a different kind, capable of giving

electricity of such high tension. In this manner, direct or alternating

currents may be converted, and in both cases the current-impulses may be

of any desired frequency. When the currents charging the condenser are

of the same direction, and it is desired that the converted currents

should also be of one direction, the resistance of the discharging

circuit should, of course, be so chosen that there are no oscillations.

[Illustration: FIG. 128.]

In operating devices on the above plan I have observed curious phenomena

of impedance which are of interest. For instance if a thick copper bar

be bent, as indicated in Fig. 128, and shunted by ordinary incandescent

lamps, then, by passing the discharge between the knobs, the lamps may

be brought to incandescence although they are short-circuited. When a

large induction coil is employed it is easy to obtain nodes on the bar,

which are rendered evident by the different degree of brilliancy of the

lamps, as shown roughly in Fig. 128. The nodes are never clearly

defined, but they are simply maxima and minima of potentials along the

bar. This is probably due to the irregularity of the arc between the

knobs. In general when the above-described plan of conversion from high

to low tension is used, the behavior of the disruptive discharge may be

closely studied. The nodes may also be investigated by means of an

ordinary Cardew voltmeter which should be well insulated. Geissler

tubes may also be lighted across the points of the bent bar; in this

case, of course, it is better to employ smaller capacities. I have found

it practicable to light up in this manner a lamp, and even a Geissler

tube, shunted by a short, heavy block of metal, and this result seems at

first very curious. In fact, the thicker the copper bar in Fig. 128, the

better it is for the success of the experiments, as they appear more

striking. When lamps with long slender filaments are used it will be

often noted that the filaments are from time to time violently vibrated,

the vibration being smallest at the nodal points. This vibration seems

to be due to an electrostatic action between the filament and the glass

of the bulb.

[Illustration: FIG. 129.]

In some of the above experiments it is preferable to use special lamps

having a straight filament as shown in Fig. 129. When such a lamp is

used a still more curious phenomenon than those described may be

observed. The lamp may be placed across the copper bar and lighted, and

by using somewhat larger capacities, or, in other words, smaller

frequencies or smaller impulsive impedances, the filament may be brought

to any desired degree of incandescence. But when the impedance is

increased, a point is reached when comparatively little current passes

through the carbon, and most of it through the rarefied gas; or perhaps

it may be more correct to state that the current divides nearly evenly

through both, in spite of the enormous difference in the resistance, and

this would be true unless the gas and the filament behave differently.

It is then noted that the whole bulb is brilliantly illuminated, and the

ends of the leading-in wires become incandescent and often throw off

sparks in consequence of the violent bombardment, but the carbon

filament remains dark. This is illustrated in Fig. 129. Instead of the

filament a single wire extending through the whole bulb may be used,

and in this case the phenomenon would seem to be still more interesting.

From the above experiment it will be evident, that when ordinary lamps

are operated by the converted currents, those should be preferably taken

in which the platinum wires are far apart, and the frequencies used

should not be too great, else the discharge will occur at the ends of

the filament or in the base of the lamp between the leading-in wires,

and the lamp might then be damaged.

In presenting to you these results of my investigation on the subject

under consideration, I have paid only a passing notice to facts upon

which I could have dwelt at length, and among many observations I have

selected only those which I thought most likely to interest you. The

field is wide and completely unexplored, and at every step a new truth

is gleaned, a novel fact observed.

How far the results here borne out are capable of practical applications

will be decided in the future. As regards the production of light, some

results already reached are encouraging and make me confident in

asserting that the practical solution of the problem lies in the

direction I have endeavored to indicate. Still, whatever may be the

immediate outcome of these experiments I am hopeful that they will only

prove a step in further development towards the ideal and final

perfection. The possibilities which are opened by modern research are so

vast that even the most reserved must feel sanguine of the future.

Eminent scientists consider the problem of utilizing one kind of

radiation without the others a rational one. In an apparatus designed

for the production of light by conversion from any form of energy into

that of light, such a result can never be reached, for no matter what

the process of producing the required vibrations, be it electrical,

chemical or any other, it will not be possible to obtain the higher

light vibrations without going through the lower heat vibrations. It is

the problem of imparting to a body a certain velocity without passing

through all lower velocities. But there is a possibility of obtaining

energy not only in the form of light, but motive power, and energy of

any other form, in some more direct way from the medium. The time will

be when this will be accomplished, and the time has come when one may

utter such words before an enlightened audience without being considered

a visionary. We are whirling through endless space with an

inconceivable speed, all around us everything is spinning, everything is

moving, everywhere is energy. There _must_ be some way of availing

ourselves of this energy more directly. Then, with the light obtained

from the medium, with the power derived from it, with every form of

energy obtained without effort, from the store forever inexhaustible,

humanity will advance with giant strides. The mere contemplation of

these magnificent possibilities expands our minds, strengthens our hopes

and fills our hearts with supreme delight.

CHAPTER XXVII.

EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH

FREQUENCY.[2]

[2] Lecture delivered before the Institution of Electrical

Engineers, London, February, 1892.

I cannot find words to express how deeply I feel the honor of addressing

some of the foremost thinkers of the present time, and so many able

scientific men, engineers and electricians, of the country greatest in

scientific achievements.

The results which I have the honor to present before such a gathering I

cannot call my own. There are among you not a few who can lay better

claim than myself on any feature of merit which this work may contain. I

need not mention many names which are world-known--names of those among

you who are recognized as the leaders in this enchanting science; but

one, at least, I must mention--a name which could not be omitted in a

demonstration of this kind. It is a name associated with the most

beautiful invention ever made: it is Crookes!

When I was at college, a good while ago, I read, in a translation (for

then I was not familiar with your magnificent language), the description

of his experiments on radiant matter. I read it only once in my

life--that time--yet every detail about that charming work I can

remember to this day. Few are the books, let me say, which can make such

an impression upon the mind of a student.

But if, on the present occasion, I mention this name as one of many your

Institution can boast of, it is because I have more than one reason to

do so. For what I have to tell you and to show you this evening

concerns, in a large measure, that same vague world which Professor

Crookes has so ably explored; and, more than this, when I trace back the

mental process which led me to these advances--which even by myself

cannot be considered trifling, since they are so appreciated by you--I

believe that their real origin, that which started me to work in this

direction, and brought me to them, after a long period of constant

thought, was that fascinating little book which I read many years ago.

And now that I have made a feeble effort to express my homage and

acknowledge my indebtedness to him and others among you, I will make a

second effort, which I hope you will not find so feeble as the first, to

entertain you.

Give me leave to introduce the subject in a few words.

A short time ago I had the honor to bring before our American Institute

of Electrical Engineers some results then arrived at by me in a novel

line of work. I need not assure you that the many evidences which I have

received that English scientific men and engineers were interested in

this work have been for me a great reward and encouragement. I will not

dwell upon the experiments already described, except with the view of

completing, or more clearly expressing, some ideas advanced by me

before, and also with the view of rendering the study here presented

self-contained, and my remarks on the subject of this evening's lecture

consistent.

This investigation, then, it goes without saying, deals with alternating

currents, and to be more precise, with alternating currents of high

potential and high frequency. Just in how much a very high frequency is

essential for the production of the results presented is a question

which, even with my present experience, would embarrass me to answer.

Some of the experiments may be performed with low frequencies; but very

high frequencies are desirable, not only on account of the many effects

secured by their use, but also as a convenient means of obtaining, in

the induction apparatus employed, the high potentials, which in their

turn are necessary to the demonstration of most of the experiments here

contemplated.

Of the various branches of electrical investigation, perhaps the most

interesting and the most immediately promising is that dealing with

alternating currents. The progress in this branch of applied science has

been so great in recent years that it justifies the most sanguine hopes.

Hardly have we become familiar with one fact, when novel experiences are

met and new avenues of research are opened. Even at this hour

possibilities not dreamed of before are, by the use of these currents,

partly realized. As in nature all is ebb and tide, all is wave motion,

so it seems that in all branches of industry alternating

currents--electric wave motion--will have the sway.

One reason, perhaps, why this branch of science is being so rapidly

developed is to be found in the interest which is attached to its

experimental study. We wind a simple ring of iron with coils; we

establish the connections to the generator, and with wonder and delight

we note the effects of strange forces which we bring into play, which

allow us to transform, to transmit and direct energy at will. We arrange

the circuits properly, and we see the mass of iron and wires behave as

though it were endowed with life, spinning a heavy armature, through

invisible connections, with great speed and power--with the energy

possibly conveyed from a great distance. We observe how the energy of an

alternating current traversing the wire manifests itself--not so much in

the wire as in the surrounding space--in the most surprising manner,

taking the forms of heat, light, mechanical energy, and, most surprising

of all, even chemical affinity. All these observations fascinate us, and

fill us with an intense desire to know more about the nature of these

phenomena. Each day we go to our work in the hope of discovering,--in

the hope that some one, no matter who, may find a solution of one of the

pending great problems,--and each succeeding day we return to our task

with renewed ardor; and even if we _are_ unsuccessful, our work has not

been in vain, for in these strivings, in these efforts, we have found

hours of untold pleasure, and we have directed our energies to the

benefit of mankind.

We may take--at random, if you choose--any of the many experiments which

may be performed with alternating currents; a few of which only, and by

no means the most striking, form the subject of this evening's

demonstration; they are all equally interesting, equally inciting to

thought.

Here is a simple glass tube from which the air has been partially

exhausted. I take hold of it; I bring my body in contact with a wire

conveying alternating currents of high potential, and the tube in my

hand is brilliantly lighted. In whatever position I may put it, wherever

I move it in space, as far as I can reach, its soft, pleasing light

persists with undiminished brightness.

Here is an exhausted bulb suspended from a single wire. Standing on an

insulated support, I grasp it, and a platinum button mounted in it is

brought to vivid incandescence.

Here, attached to a leading wire, is another bulb, which, as I touch its

metallic socket, is filled with magnificent colors of phosphorescent

light.

Here still another, which by my fingers' touch casts a shadow--the

Crookes shadow--of the stem inside of it.

Here, again, insulated as I stand on this platform, I bring my body in

contact with one of the terminals of the secondary of this induction

coil--with the end of a wire many miles long--and you see streams of

light break forth from its distant end, which is set in violent

vibration.

Here, once more, I attach these two plates of wire gauze to the

terminals of the coil; I set them a distance apart, and I set the coil

to work. You may see a small spark pass between the plates. I insert a

thick plate of one of the best dielectrics between them, and instead of

rendering altogether impossible, as we are used to expect, I _aid_ the

passage of the discharge, which, as I insert the plate, merely changes

in appearance and assumes the form of luminous streams.

Is there, I ask, can there be, a more interesting study than that of

alternating currents?

In all these investigations, in all these experiments, which are so

very, very interesting, for many years past--ever since the greatest

experimenter who lectured in this hall discovered its principle--we have

had a steady companion, an appliance familiar to every one, a plaything

once, a thing of momentous importance now--the induction coil. There is

no dearer appliance to the electrician. From the ablest among you, I

dare say, down to the inexperienced student, to your lecturer, we all

have passed many delightful hours in experimenting with the induction

coil. We have watched its play, and thought and pondered over the

beautiful phenomena which it disclosed to our ravished eyes. So well

known is this apparatus, so familiar are these phenomena to every one,

that my courage nearly fails me when I think that I have ventured to

address so able an audience, that I have ventured to entertain you with

that same old subject. Here in reality is the same apparatus, and here

are the same phenomena, only the apparatus is operated somewhat

differently, the phenomena are presented in a different aspect. Some of

the results we find as expected, others surprise us, but all captivate

our attention, for in scientific investigation each novel result

achieved may be the centre of a new departure, each novel fact learned

may lead to important developments.

Usually in operating an induction coil we have set up a vibration of

moderate frequency in the primary, either by means of an interrupter or

break, or by the use of an alternator. Earlier English investigators, to

mention only Spottiswoode and J. E. H. Gordon, have used a rapid break

in connection with the coil. Our knowledge and experience of to-day

enables us to see clearly why these coils under the conditions of the

test did not disclose any remarkable phenomena, and why able

experimenters failed to perceive many of the curious effects which have

since been observed.

In the experiments such as performed this evening, we operate the coil

either from a specially constructed alternator capable of giving many

thousands of reversals of current per second, or, by disruptively

discharging a condenser through the primary, we set up a vibration in

the secondary circuit of a frequency of many hundred thousand or

millions per second, if we so desire; and in using either of these means

we enter a field as yet unexplored.

It is impossible to pursue an investigation in any novel line without

finally making some interesting observation or learning some useful

fact. That this statement is applicable to the subject of this lecture

the many curious and unexpected phenomena which we observe afford a

convincing proof. By way of illustration, take for instance the most

obvious phenomena, those of the discharge of the induction coil.

Here is a coil which is operated by currents vibrating with extreme

rapidity, obtained by disruptively discharging a Leyden jar. It would

not surprise a student were the lecturer to say that the secondary of

this coil consists of a small length of comparatively stout wire; it

would not surprise him were the lecturer to state that, in spite of

this, the coil is capable of giving any potential which the best

insulation of the turns is able to withstand; but although he may be

prepared, and even be indifferent as to the anticipated result, yet the

aspect of the discharge of the coil will surprise and interest him.

Every one is familiar with the discharge of an ordinary coil; it need

not be reproduced here. But, by way of contrast, here is a form of

discharge of a coil, the primary current of which is vibrating several

hundred thousand times per second. The discharge of an ordinary coil

appears as a simple line or band of light. The discharge of this coil

appears in the form of powerful brushes and luminous streams issuing

from all points of the two straight wires attached to the terminals of

the secondary. (Fig. 130.)

[Illustration: FIG. 130.]

[Illustration: FIG. 131.]

Now compare this phenomenon which you have just witnessed with the

discharge of a Holtz or Wimshurst machine--that other interesting

appliance so dear to the experimenter. What a difference there is

between these phenomena! And yet, had I made the necessary

arrangements--which could have been made easily, were it not that they

would interfere with other experiments--I could have produced with this

coil sparks which, had I the coil hidden from your view and only two

knobs exposed, even the keenest observer among you would find it

difficult, if not impossible, to distinguish from those of an influence

or friction machine. This may be done in many ways--for instance, by

operating the induction coil which charges the condenser from an

alternating-current machine of very low frequency, and preferably

adjusting the discharge circuit so that there are no oscillations set up

in it. We then obtain in the secondary circuit, if the knobs are of the

required size and properly set, a more or less rapid succession of

sparks of great intensity and small quantity, which possess the same

brilliancy, and are accompanied by the same sharp crackling sound, as

those obtained from a friction or influence machine.

Another way is to pass through two primary circuits, having a common

secondary, two currents of a slightly different period, which produce in

the secondary circuit sparks occurring at comparatively long intervals.

But, even with the means at hand this evening, I may succeed in

imitating the spark of a Holtz machine. For this purpose I establish

between the terminals of the coil which charges the condenser a long,

unsteady arc, which is periodically interrupted by the upward current of

air produced by it. To increase the current of air I place on each side

of the arc, and close to it, a large plate of mica. The condenser

charged from this coil discharges into the primary circuit of a second

coil through a small air gap, which is necessary to produce a sudden

rush of current through the primary. The scheme of connections in the

present experiment is indicated in Fig. 131.

G is an ordinarily constructed alternator, supplying the primary P of an

induction coil, the secondary S of which charges the condensers or jars

C C. The terminals of the secondary are connected to the inside coatings

of the jars, the outer coatings being connected to the ends of the

primary _p p_ of a second induction coil. This primary _p p_ has a small

air gap _a b_.

The secondary _s_ of this coil is provided with knobs or spheres K K of

the proper size and set at a distance suitable for the experiment.

A long arc is established between the terminals A B of the first

induction coil. M M are the mica plates.

Each time the arc is broken between A and B the jars are quickly charged

and discharged through the primary _p p_, producing a snapping spark

between the knobs K K. Upon the arc forming between A and B the

potential falls, and the jars cannot be charged to such high potential

as to break through the air gap _a b_ until the arc is again broken by

the draught.

In this manner sudden impulses, at long intervals, are produced in the

primary _p p_, which in the secondary _s_ give a corresponding number of

impulses of great intensity. If the secondary knobs or spheres, K K, are

of the proper size, the sparks show much resemblance to those of a Holtz

machine.

But these two effects, which to the eye appear so very different, are

only two of the many discharge phenomena. We only need to change the

conditions of the test, and again we make other observations of

interest.

When, instead of operating the induction coil as in the last two

experiments, we operate it from a high frequency alternator, as in the

next experiment, a systematic study of the phenomena is rendered much

more easy. In such case, in varying the strength and frequency of the

currents through the primary, we may observe five distinct forms of

discharge, which I have described in my former paper on the subject

before the American Institute of Electrical Engineers, May 20, 1891.

It would take too much time, and it would lead us too far from the

subject presented this evening, to reproduce all these forms, but it

seems to me desirable to show you one of them. It is a brush discharge,

which is interesting in more than one respect. Viewed from a near

position it resembles much a jet of gas escaping under great pressure.

We know that the phenomenon is due to the agitation of the molecules

near the terminal, and we anticipate that some heat must be developed by

the impact of the molecules against the terminal or against each other.

Indeed, we find that the brush is hot, and only a little thought leads

us to the conclusion that, could we but reach sufficiently high

frequencies, we could produce a brush which would give intense light and

heat, and which would resemble in every particular an ordinary flame,

save, perhaps, that both phenomena might not be due to the same

agent--save, perhaps, that chemical affinity might not be _electrical_

in its nature.

As the production of heat and light is here due to the impact of the

molecules, or atoms of air, or something else besides, and, as we can

augment the energy simply by raising the potential, we might, even with

frequencies obtained from a dynamo machine, intensify the action to such

a degree as to bring the terminal to melting heat. But with such low

frequencies we would have to deal always with something of the nature of

an electric current. If I approach a conducting object to the brush, a

thin little spark passes, yet, even with the frequencies used this

evening, the tendency to spark is not very great. So, for instance, if I

hold a metallic sphere at some distance above the terminal, you may see

the whole space between the terminal and sphere illuminated by the

streams without the spark passing; and with the much higher frequencies

obtainable by the disruptive discharge of a condenser, were it not for

the sudden impulses, which are comparatively few in number, sparking

would not occur even at very small distances. However, with incomparably

higher frequencies, which we may yet find means to produce efficiently,

and provided that electric impulses of such high frequencies could be

transmitted through a conductor, the electrical characteristics of the

brush discharge would completely vanish--no spark would pass, no shock

would be felt--yet we would still have to deal with an _electric_

phenomenon, but in the broad, modern interpretation of the word. In my

first paper, before referred to, I have pointed out the curious

properties of the brush, and described the best manner of producing it,

but I have thought it worth while to endeavor to express myself more

clearly in regard to this phenomenon, because of its absorbing interest.

When a coil is operated with currents of very high frequency, beautiful

brush effects may be produced, even if the coil be of comparatively

small dimensions. The experimenter may vary them in many ways, and, if

it were for nothing else, they afford a pleasing sight. What adds to

their interest is that they may be produced with one single terminal as

well as with two--in fact, often better with one than with two.

But of all the discharge phenomena observed, the most pleasing to the

eye, and the most instructive, are those observed with a coil which is

operated by means of the disruptive discharge of a condenser. The power

of the brushes, the abundance of the sparks, when the conditions are

patiently adjusted, is often amazing. With even a very small coil, if it

be so well insulated as to stand a difference of potential of several

thousand volts per turn, the sparks may be so abundant that the whole

coil may appear a complete mass of fire.

Curiously enough the sparks, when the terminals of the coil are set at a

considerable distance, seem to dart in every possible direction as

though the terminals were perfectly independent of each other. As the

sparks would soon destroy the insulation, it is necessary to prevent

them. This is best done by immersing the coil in a good liquid

insulator, such as boiled-out oil. Immersion in a liquid may be

considered almost an absolute necessity for the continued and successful

working of such a coil.

It is, of course, out of the question, in an experimental lecture, with

only a few minutes at disposal for the performance of each experiment,

to show these discharge phenomena to advantage, as, to produce each

phenomenon at its best, a very careful adjustment is required. But even

if imperfectly produced, as they are likely to be this evening, they are

sufficiently striking to interest an intelligent audience.

Before showing some of these curious effects I must, for the sake of

completeness, give a short description of the coil and other apparatus

used in the experiments with the disruptive discharge this evening.

[Illustration: FIG. 132.]

It is contained in a box B (Fig. 132) of thick boards of hard wood,

covered on the outside with a zinc sheet Z, which is carefully soldered

all around. It might be advisable, in a strictly scientific

investigation, when accuracy is of great importance, to do away with the

metal cover, as it might introduce many errors, principally on account

of its complex action upon the coil, as a condenser of very small

capacity and as an electrostatic and electromagnetic screen. When the

coil is used for such experiments as are here contemplated, the

employment of the metal cover offers some practical advantages, but

these are not of sufficient importance to be dwelt upon.

The coil should be placed symmetrically to the metal cover, and the

space between should, of course, not be too small, certainly not less

than, say, five centimetres, but much more if possible; especially the

two sides of the zinc box, which are at right angles to the axis of the

coil, should be sufficiently remote from the latter, as otherwise they

might impair its action and be a source of loss.

The coil consists of two spools of hard rubber R R, held apart at a

distance of 10 centimetres by bolts C and nuts _n_, likewise of hard

rubber. Each spool comprises a tube T of approximately 8 centimetres

inside diameter, and 3 millimetres thick, upon which are screwed two

flanges F F, 24 centimetres square, the space between the flanges being

about 3 centimetres. The secondary, S S, of the best gutta

percha-covered wire, has 26 layers, 10 turns in each, giving for each

half a total of 260 turns. The two halves are wound oppositely and

connected in series, the connection between both being made over the

primary. This disposition, besides being convenient, has the advantage

that when the coil is well balanced--that is, when both of its

terminals T_{1}, T_{1}, are connected to bodies or devices of equal

capacity--there is not much danger of breaking through to the primary,

and the insulation between the primary and the secondary need not be

thick. In using the coil it is advisable to attach to _both_ terminals

devices of nearly equal capacity, as, when the capacity of the terminals

is not equal, sparks will be apt to pass to the primary. To avoid this,

the middle point of the secondary may be connected to the primary, but

this is not always practicable.

The primary P P is wound in two parts, and oppositely, upon a wooden

spool w, and the four ends are led out of the oil through hard rubber

tubes _t t_. The ends of the secondary T_{1} T_{1}, are also led out of

the oil through rubber tubes t_{1} t_{1} of great thickness. The

primary and secondary layers are insulated by cotton cloth, the

thickness of the insulation, of course, bearing some proportion to the

difference of potential between the turns of the different layers. Each

half of the primary has four layers, 24 turns in each, this giving a

total of 96 turns. When both the parts are connected in series, this

gives a ratio of conversion of about 1:2.7, and with the primaries in

multiple, 1:5.4; but in operating with very rapidly alternating currents

this ratio does not convey even an approximate idea of the ratio of the

E. M. F's. in the primary and secondary circuits. The coil is held in

position in the oil on wooden supports, there being about 5 centimetres

thickness of oil all round. Where the oil is not specially needed, the

space is filled with pieces of wood, and for this purpose principally

the wooden box B surrounding the whole is used.

The construction here shown is, of course, not the best on general

principles, but I believe it is a good and convenient one for the

production of effects in which an excessive potential and a very small

current are needed.

In connection with the coil I use either the ordinary form of discharger

or a modified form. In the former I have introduced two changes which

secure some advantages, and which are obvious. If they are mentioned, it

is only in the hope that some experimenter may find them of use.

One of the changes is that the adjustable knobs A and B (Fig. 133), of

the discharger are held in jaws of brass, J J, by spring pressure, this

allowing of turning them successively into different positions, and so

doing away with the tedious process of frequent polishing up.

[Illustration: FIG. 133.]

The other change consists in the employment of a strong electromagnet

N S, which is placed with its axis at right angles to the line joining

the knobs A and B, and produces a strong magnetic field between them.

The pole pieces of the magnet are movable and properly formed so as to

protrude between the brass knobs, in order to make the field as intense

as possible; but to prevent the discharge from jumping to the magnet the

pole pieces are protected by a layer of mica, M M, of sufficient

thickness; s_{1} s_{1} and s_{2} s_{2} are screws for fastening the

wires. On each side one of the screws is for large and the other for

small wires. L L are screws for fixing in position the rods R R, which

support the knobs.

In another arrangement with the magnet I take the discharge between the

rounded pole pieces themselves, which in such case are insulated and

preferably provided with polished brass caps.

The employment of an intense magnetic field is of advantage principally

when the induction coil or transformer which charges the condenser is

operated by currents of very low frequency. In such a case the number of

the fundamental discharges between the knobs may be so small as to

render the currents produced in the secondary unsuitable for many

experiments. The intense magnetic field then serves to blow out the arc

between the knobs as soon as it is formed, and the fundamental

discharges occur in quicker succession.

[Illustration: FIG. 134.]

Instead of the magnet, a draught or blast of air may be employed with

some advantage. In this case the arc is preferably established between

the knobs A B, in Fig. 131 (the knobs _a b_ being generally joined, or

entirely done away with), as in this disposition the arc is long and

unsteady, and is easily affected by the draught.

When a magnet is employed to break the arc, it is better to choose the

connection indicated diagrammatically in Fig. 134, as in this case the

currents forming the arc are much more powerful, and the magnetic field

exercises a greater influence. The use of the magnet permits, however,

of the arc being replaced by a vacuum tube, but I have encountered great

difficulties in working with an exhausted tube.

The other form of discharger used in these and similar experiments is

indicated in Figs. 135 and 136. It consists of a number of brass pieces

_c c_ (Fig. 135), each of which comprises a spherical middle portion _m_

with an extension _e_ below--which is merely used to fasten the piece in

a lathe when polishing up the discharging surface--and a column above,

which consists of a knurled flange _f_ surmounted by a threaded stem _l_

carrying a nut _n_, by means of which a wire is fastened to the column.

The flange _f_ conveniently serves for holding the brass piece when

fastening the wire, and also for turning it in any position when it

becomes necessary to present a fresh discharging surface. Two stout

strips of hard rubber R R, with planed grooves _g g_ (Fig. 136) to fit

the middle portion of the pieces _c c_, serve to clamp the latter and

hold them firmly in position by means of two bolts C C (of which only

one is shown) passing through the ends of the strips.

[Illustration: FIG. 135.]

[Illustration: FIG. 136.]

In the use of this kind of discharger I have found three principal

advantages over the ordinary form. First, the dielectric strength of a

given total width of air space is greater when a great many small air

gaps are used instead of one, which permits of working with a smaller

length of air gap, and that means smaller loss and less deterioration of

the metal; secondly, by reason of splitting the arc up into smaller

arcs, the polished surfaces are made to last much longer; and, thirdly,

the apparatus affords some gauge in the experiments. I usually set the

pieces by putting between them sheets of uniform thickness at a certain

very small distance which is known from the experiments of Sir William

Thomson to require a certain electromotive force to be bridged by the

spark.

It should, of course, be remembered that the sparking distance is much

diminished as the frequency is increased. By taking any number of spaces

the experimenter has a rough idea of the electromotive force, and he

finds it easier to repeat an experiment, as he has not the trouble of

setting the knobs again and again. With this kind of discharger I have

been able to maintain an oscillating motion without any spark being

visible with the naked eye between the knobs, and they would not show a

very appreciable rise in temperature. This form of discharge also lends

itself to many arrangements of condensers and circuits which are often

very convenient and time-saving. I have used it preferably in a

disposition similar to that indicated in Fig. 131, when the currents

forming the arc are small.

I may here mention that I have also used dischargers with single or

multiple air gaps, in which the discharge surfaces were rotated with

great speed. No particular advantage was, however, gained by this

method, except in cases where the currents from the condenser were large

and the keeping cool of the surfaces was necessary, and in cases when,

the discharge not being oscillating of itself, the arc as soon as

established was broken by the air current, thus starting the vibration

at intervals in rapid succession. I have also used mechanical

interrupters in many ways. To avoid the difficulties with frictional

contacts, the preferred plan adopted was to establish the arc and rotate

through it at great speed a rim of mica provided with many holes and

fastened to a steel plate. It is understood, of course, that the

employment of a magnet, air current, or other interrupter, produces no

effect worth noticing, unless the self-induction, capacity and

resistance are so related that there are oscillations set up upon each

interruption.

I will now endeavor to show you some of the most noteworthy of these

discharge phenomena.

I have stretched across the room two ordinary cotton covered wires, each

about seven metres in length. They are supported on insulating cords at

a distance of about thirty centimetres. I attach now to each of the

terminals of the coil one of the wires, and set the coil in action.

Upon turning the lights off in the room you see the wires strongly

illuminated by the streams issuing abundantly from their whole surface

in spite of the cotton covering, which may even be very thick. When the

experiment is performed under good conditions, the light from the wires

is sufficiently intense to allow distinguishing the objects in a room.

To produce the best result it is, of course, necessary to adjust

carefully the capacity of the jars, the arc between the knobs and the

length of the wires. My experience is that calculation of the length of

the wires leads, in such case, to no result whatever. The experimenter

will do best to take the wires at the start very long, and then adjust

by cutting off first long pieces, and then smaller and smaller ones as

he approaches the right length.

A convenient way is to use an oil condenser of very small capacity,

consisting of two small adjustable metal plates, in connection with this

and similar experiments. In such case I take wires rather short and at

the beginning set the condenser plates at maximum distance. If the

streams from the wires increase by approach of the plates, the length of

the wires is about right; if they diminish, the wires are too long for

that frequency and potential. When a condenser is used in connection

with experiments with such a coil, it should be an oil condenser by all

means, as in using an air condenser considerable energy might be wasted.

The wires leading to the plates in the oil should be very thin, heavily

coated with some insulating compound, and provided with a conducting

covering--this preferably extending under the surface of the oil. The

conducting cover should not be too near the terminals, or ends, of the

wire, as a spark would be apt to jump from the wire to it. The

conducting coating is used to diminish the air losses, in virtue of its

action as an electrostatic screen. As to the size of the vessel

containing the oil, and the size of the plates, the experimenter gains

at once an idea from a rough trial. The size of the plates _in oil_ is,

however, calculable, as the dielectric losses are very small.

In the preceding experiment it is of considerable interest to know what

relation the quantity of the light emitted bears to the frequency and

potential of the electric impulses. My opinion is that the heat as well

as light effects produced should be proportionate, under otherwise equal

conditions of test, to the product of frequency and square of potential,

but the experimental verification of the law, whatever it may be, would

be exceedingly difficult. One thing is certain, at any rate, and that

is, that in augmenting the potential and frequency we rapidly intensify

the streams; and, though it may be very sanguine, it is surely not

altogether hopeless to expect that we may succeed in producing a

practical illuminant on these lines. We would then be simply using

burners or flames, in which there would be no chemical process, no

consumption of material, but merely a transfer of energy, and which

would, in all probability, emit more light and less heat than ordinary

flames.

[Illustration: FIG. 137.]

The luminous intensity of the streams is, of course, considerably

increased when they are focused upon a small surface. This may be shown

by the following experiment:

I attach to one of the terminals of the coil a wire _w_ (Fig. 137), bent

in a circle of about 30 centimetres in diameter, and to the other

terminal I fasten a small brass sphere _s_, the surface of the wire

being preferably equal to the surface of the sphere, and the centre of

the latter being in a line at right angles to the plane of the wire

circle and passing through its centre. When the discharge is established

under proper conditions, a luminous hollow cone is formed, and in the

dark one-half of the brass sphere is strongly illuminated, as shown in

the cut.

By some artifice or other it is easy to concentrate the streams upon

small surfaces and to produce very strong light effects. Two thin wires

may thus be rendered intensely luminous.

In order to intensify the streams the wires should be very thin and

short; but as in this case their capacity would be generally too small

for the coil--at least for such a one as the present--it is necessary to

augment the capacity to the required value, while, at the same time, the

surface of the wires remains very small. This may be done in many ways.

[Illustration: FIG. 138.]

Here, for instance, I have two plates, R R, of hard rubber (Fig. 138),

upon which I have glued two very thin wires _w w_, so as to form a name.

The wires may be bare or covered with the best insulation--it is

immaterial for the success of the experiment. Well insulated wires, if

anything, are preferable. On the back of each plate, indicated by the

shaded portion, is a tinfoil coating _t t_. The plates are placed in

line at a sufficient distance to prevent a spark passing from one wire

to the other. The two tinfoil coatings I have joined by a conductor C,

and the two wires I presently connect to the terminals of the coil. It

is now easy, by varying the strength and frequency of the currents

through the primary, to find a point at which the capacity of the system

is best suited to the conditions, and the wires become so strongly

luminous that, when the light in the room is turned off the name formed

by them appears in brilliant letters.

It is perhaps preferable to perform this experiment with a coil operated

from an alternator of high frequency, as then, owing to the harmonic

rise and fall, the streams are very uniform, though they are less

abundant than when produced with such a coil as the present one. This

experiment, however, may be performed with low frequencies, but much

less satisfactorily.

[Illustration: FIG. 139.]

When two wires, attached to the terminals of the coil, are set at the

proper distance, the streams between them may be so intense as to

produce a continuous luminous sheet. To show this phenomenon I have here

two circles, C and _c_ (Fig. 139), of rather stout wire, one being about

80 centimetres and the other 30 centimetres in diameter. To each of the

terminals of the coil I attach one of the circles. The supporting wires

are so bent that the circles may be placed in the same plane, coinciding

as nearly as possible. When the light in the room is turned off and the

coil set to work, you see the whole space between the wires uniformly

filled with streams, forming a luminous disc, which could be seen from a

considerable distance, such is the intensity of the streams. The outer

circle could have been much larger than the present one; in fact, with

this coil I have used much larger circles, and I have been able to

produce a strongly luminous sheet, covering an area of more than one

square metre, which is a remarkable effect with this very small coil. To

avoid uncertainty, the circle has been taken smaller, and the area is

now about 0.43 square metre.

The frequency of the vibration, and the quickness of succession of the

sparks between the knobs, affect to a marked degree the appearance of

the streams. When the frequency is very low, the air gives way in more

or less the same manner, as by a steady difference of potential, and the

streams consist of distinct threads, generally mingled with thin sparks,

which probably correspond to the successive discharges occurring between

the knobs. But when the frequency is extremely high, and the arc of the

discharge produces a very _loud_ and _smooth_ sound--showing both that

oscillation takes place and that the sparks succeed each other with

great rapidity--then the luminous streams formed are perfectly uniform.

To reach this result very small coils and jars of small capacity should

be used. I take two tubes of thick Bohemian glass, about 5 centimetres

in diameter and 20 centimetres long. In each of the tubes I slip a

primary of very thick copper wire. On the top of each tube I wind a

secondary of much thinner gutta-percha covered wire. The two secondaries

I connect in series, the primaries preferably in multiple arc. The tubes

are then placed in a large glass vessel, at a distance of 10 to 15

centimetres from each other, on insulating supports, and the vessel is

filled with boiled-out oil, the oil reaching about an inch above the

tubes. The free ends of the secondary are lifted out of the coil and

placed parallel to each other at a distance of about ten centimetres.

The ends which are scraped should be dipped in the oil. Two four-pint

jars joined in series may be used to discharge through the primary. When

the necessary adjustments in the length and distance of the wires above

the oil and in the arc of discharge are made, a luminous sheet is

produced between the wires which is perfectly smooth and textureless,

like the ordinary discharge through a moderately exhausted tube.

I have purposely dwelt upon this apparently insignificant experiment. In

trials of this kind the experimenter arrives at the startling conclusion

that, to pass ordinary luminous discharges through gases, no particular

degree of exhaustion is needed, but that the gas may be at ordinary or

even greater pressure. To accomplish this, a very high frequency is

essential; a high potential is likewise required, but this is merely an

incidental necessity. These experiments teach us that, in endeavoring to

discover novel methods of producing light by the agitation of atoms, or

molecules, of a gas, we need not limit our research to the vacuum tube,

but may look forward quite seriously to the possibility of obtaining the

light effects without the use of any vessel whatever, with air at

ordinary pressure.

Such discharges of very high frequency, which render luminous the air at

ordinary pressures, we have probably occasion often to witness in

Nature. I have no doubt that if, as many believe, the aurora borealis is

produced by sudden cosmic disturbances, such as eruptions at the sun's

surface, which set the electrostatic charge of the earth in an extremely

rapid vibration, the red glow observed is not confined to the upper

rarefied strata of the air, but the discharge traverses, by reason of

its very high frequency, also the dense atmosphere in the form of a

_glow_, such as we ordinarily produce in a slightly exhausted tube. If

the frequency were very low, or even more so, if the charge were not at

all vibrating, the dense air would break down as in a lightning

discharge. Indications of such breaking down of the lower dense strata

of the air have been repeatedly observed at the occurrence of this

marvelous phenomenon; but if it does occur, it can only be attributed to

the fundamental disturbances, which are few in number, for the vibration

produced by them would be far too rapid to allow a disruptive break. It

is the original and irregular impulses which affect the instruments; the

superimposed vibrations probably pass unnoticed.

When an ordinary low frequency discharge is passed through moderately

rarefied air, the air assumes a purplish hue. If by some means or other

we increase the intensity of the molecular, or atomic, vibration, the

gas changes to a white color. A similar change occurs at ordinary

pressures with electric impulses of very high frequency. If the

molecules of the air around a wire are moderately agitated, the brush

formed is reddish or violet; if the vibration is rendered sufficiently

intense, the streams become white. We may accomplish this in various

ways. In the experiment before shown with the two wires across the room,

I have endeavored to secure the result by pushing to a high value both

the frequency and potential; in the experiment with the thin wires glued

on the rubber plate I have concentrated the action upon a very small

surface--in other words, I have worked with a great electric density.

[Illustration: FIG. 140.]

A most curious form of discharge is observed with such a coil when the

frequency and potential are pushed to the extreme limit. To perform the

experiment, every part of the coil should be heavily insulated, and only

two small spheres--or, better still, two sharp-edged metal discs (_d d_,

Fig. 140) of no more than a few centimetres in diameter--should be

exposed to the air. The coil here used is immersed in oil, and the ends

of the secondary reaching out of the oil are covered with an air-tight

cover of hard rubber of great thickness. All cracks, if there are any,

should be carefully stopped up, so that the brush discharge cannot form

anywhere except on the small spheres or plates which are exposed to the

air. In this case, since there are no large plates or other bodies of

capacity attached to the terminals, the coil is capable of an extremely

rapid vibration. The potential may be raised by increasing, as far as

the experimenter judges proper, the rate of change of the primary

current. With a coil not widely differing from the present, it is best

to connect the two primaries in multiple arc; but if the secondary

should have a much greater number of turns the primaries should

preferably be used in series, as otherwise the vibration might be too

fast for the secondary. It occurs under these conditions that misty

white streams break forth from the edges of the discs and spread out

phantom-like into space. With this coil, when fairly well produced, they

are about 25 to 30 centimetres long. When the hand is held against them

no sensation is produced, and a spark, causing a shock, jumps from the

terminal only upon the hand being brought much nearer. If the

oscillation of the primary current is rendered intermittent by some

means or other, there is a corresponding throbbing of the streams, and

now the hand or other conducting object may be brought in still greater

proximity to the terminal without a spark being caused to jump.

Among the many beautiful phenomena which may be produced with such a

coil, I have here selected only those which appear to possess some

features of novelty, and lead us to some conclusions of interest. One

will not find it at all difficult to produce in the laboratory, by means

of it, many other phenomena which appeal to the eye even more than these

here shown, but present no particular feature of novelty.

Early experimenters describe the display of sparks produced by an

ordinary large induction coil upon an insulating plate separating the

terminals. Quite recently Siemens performed some experiments in which

fine effects were obtained, which were seen by many with interest. No

doubt large coils, even if operated with currents of low frequencies,

are capable of producing beautiful effects. But the largest coil ever

made could not, by far, equal the magnificent display of streams and

sparks obtained from such a disruptive discharge coil when properly

adjusted. To give an idea, a coil such as the present one will cover

easily a plate of one metre in diameter completely with the streams. The

best way to perform such experiments is to take a very thin rubber or a

glass plate and glue on one side of it a narrow ring of tinfoil of very

large diameter, and on the other a circular washer, the centre of the

latter coinciding with that of the ring, and the surfaces of both being

preferably equal, so as to keep the coil well balanced. The washer and

ring should be connected to the terminals by heavily insulated thin

wires. It is easy in observing the effect of the capacity to produce a

sheet of uniform streams, or a fine network of thin silvery threads, or

a mass of loud brilliant sparks, which completely cover the plate.

Since I have advanced the idea of the conversion by means of the

disruptive discharge, in my paper before the American Institute of

Electrical Engineers at the beginning of the past year, the interest

excited in it has been considerable. It affords us a means for producing

any potentials by the aid of inexpensive coils operated from ordinary

systems of distribution, and--what is perhaps more appreciated--it

enables us to convert currents of any frequency into currents of any

other lower or higher frequency. But its chief value will perhaps be

found in the help which it will afford us in the investigations of the

phenomena of phosphorescence, which a disruptive discharge coil is

capable of exciting in innumerable cases where ordinary coils, even the

largest, would utterly fail.

Considering its probable uses for many practical purposes, and its

possible introduction into laboratories for scientific research, a few

additional remarks as to the construction of such a coil will perhaps

not be found superfluous.

It is, of course, absolutely necessary to employ in such a coil wires

provided with the best insulation.

Good coils may be produced by employing wires covered with several

layers of cotton, boiling the coil a long time in pure wax, and cooling

under moderate pressure. The advantage of such a coil is that it can be

easily handled, but it cannot probably give as satisfactory results as a

coil immersed in pure oil. Besides, it seems that the presence of a

large body of wax affects the coil disadvantageously, whereas this does

not seem to be the case with oil. Perhaps it is because the dielectric

losses in the liquid are smaller.

I have tried at first silk and cotton covered wires with oil immersions,

but I have been gradually led to use gutta-percha covered wires, which

proved most satisfactory. Gutta-percha insulation adds, of course, to

the capacity of the coil, and this, especially if the coil be large, is

a great disadvantage when extreme frequencies are desired; but, on the

other hand, gutta-percha will withstand much more than an equal

thickness of oil, and this advantage should be secured at any price.

Once the coil has been immersed, it should never be taken out of the oil

for more than a few hours, else the gutta-percha will crack up and the

coil will not be worth half as much as before. Gutta-percha is probably

slowly attacked by the oil, but after an immersion of eight to nine

months I have found no ill effects.

I have obtained two kinds of gutta-percha wire known in commerce: in one

the insulation sticks tightly to the metal, in the other it does not.

Unless a special method is followed to expel all air, it is much safer

to use the first kind. I wind the coil within an oil tank so that all

interstices are filled up with the oil. Between the layers I use cloth

boiled out thoroughly in oil, calculating the thickness according to the

difference of potential between the turns. There seems not to be a very

great difference whatever kind of oil is used; I use paraffine or

linseed oil.