easily practicable with small coils, is the following: Construct a box
of hardwood of very thick boards which have been for a long time boiled
in oil. The boards should be so joined as to safely withstand the
external air pressure. The coil being placed and fastened in position
within the box, the latter is closed with a strong lid, and covered with
closely fitting metal sheets, the joints of which are soldered very
carefully. On the top two small holes are drilled, passing through the
metal sheet and the wood, and in these holes two small glass tubes are
inserted and the joints made air-tight. One of the tubes is connected to
a vacuum pump, and the other with a vessel containing a sufficient
quantity of boiled-out oil. The latter tube has a very small hole at the
bottom, and is provided with a stopcock. When a fairly good vacuum has
been obtained, the stopcock is opened and the oil slowly fed in.
Proceeding in this manner, it is impossible that any big bubbles, which
are the principal danger, should remain between the turns. The air is
most completely excluded, probably better than by boiling out, which,
however, when gutta-percha coated wires are used, is not practicable.
For the primaries I use ordinary line wire with a thick cotton coating.
Strands of very thin insulated wires properly interlaced would, of
course, be the best to employ for the primaries, but they are not to be
had.
In an experimental coil the size of the wires is not of great
importance. In the coil here used the primary is No. 12 and the
secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
varied considerably. It would only imply different adjustments; the
results aimed at would not be materially affected.
I have dwelt at some length upon the various forms of brush discharge
because, in studying them, we not only observe phenomena which please
our eye, but also afford us food for thought, and lead us to conclusions
of practical importance. In the use of alternating currents of very high
tension, too much precaution cannot be taken to prevent the brush
discharge. In a main conveying such currents, in an induction coil or
transformer, or in a condenser, the brush discharge is a source of great
danger to the insulation. In a condenser, especially, the gaseous matter
must be most carefully expelled, for in it the charged surfaces are
near each other, and if the potentials are high, just as sure as a
weight will fall if let go, so the insulation will give way if a single
gaseous bubble of some size be present, whereas, if all gaseous matter
were carefully excluded, the condenser would safely withstand a much
higher difference of potential. A main conveying alternating currents of
very high tension may be injured merely by a blow hole or small crack in
the insulation, the more so as a blowhole is apt to contain gas at low
pressure; and as it appears almost impossible to completely obviate such
little imperfections, I am led to believe that in our future
distribution of electrical energy by currents of very high tension,
liquid insulation will be used. The cost is a great drawback, but if we
employ an oil as an insulator the distribution of electrical energy with
something like 100,000 volts, and even more, becomes, at least with
higher frequencies, so easy that it could be hardly called an
engineering feat. With oil insulation and alternate current motors,
transmissions of power can be affected with safety and upon an
industrial basis at distances of as much as a thousand miles.
A peculiar property of oils, and liquid insulation in general, when
subjected to rapidly changing electric stresses, is to disperse any
gaseous bubbles which may be present, and diffuse them through its mass,
generally long before any injurious break can occur. This feature may be
easily observed with an ordinary induction coil by taking the primary
out, plugging up the end of the tube upon which the secondary is wound,
and filling it with some fairly transparent insulator, such as paraffine
oil. A primary of a diameter something like six millimetres smaller than
the inside of the tube may be inserted in the oil. When the coil is set
to work one may see, looking from the top through the oil, many luminous
points--air bubbles which are caught by inserting the primary, and which
are rendered luminous in consequence of the violent bombardment. The
occluded air, by its impact against the oil, heats it; the oil begins to
circulate, carrying some of the air along with it, until the bubbles are
dispersed and the luminous points disappear. In this manner, unless
large bubbles are occluded in such way that circulation is rendered
impossible, a damaging break is averted, the only effect being a
moderate warming up of the oil. If, instead of the liquid, a solid
insulation, no matter how thick, were used, a breaking through and
injury of the apparatus would be inevitable.
The exclusion of gaseous matter from any apparatus in which the
dielectric is subjected to more or less rapidly changing electric forces
is, however, not only desirable in order to avoid a possible injury of
the apparatus, but also on account of economy. In a condenser, for
instance, as long as only a solid or only a liquid dielectric is used,
the loss is small; but if a gas under ordinary or small pressure be
present the loss may be very great. Whatever the nature of the force
acting in the dielectric may be, it seems that in a solid or liquid the
molecular displacement produced by the force is small: hence the product
of force and displacement is insignificant, unless the force be very
great; but in a gas the displacement, and therefore this product, is
considerable; the molecules are free to move, they reach high speeds,
and the energy of their impact is lost in heat or otherwise. If the gas
be strongly compressed, the displacement due to the force is made
smaller, and the losses are reduced.
In most of the succeeding experiments I prefer, chiefly on account of
the regular and positive action, to employ the alternator before
referred to. This is one of the several machines constructed by me for
the purpose of these investigations. It has 384 pole projections, and is
capable of giving currents of a frequency of about 10,000 per second.
This machine has been illustrated and briefly described in my first
paper before the American Institute of Electrical Engineers, May 20th,
1891, to which I have already referred. A more detailed description,
sufficient to enable any engineer to build a similar machine, will be
found in several electrical journals of that period.
The induction coils operated from the machine are rather small,
containing from 5,000 to 15,000 turns in the secondary. They are
immersed in boiled-out linseed oil, contained in wooden boxes covered
with zinc sheet.
I have found it advantageous to reverse the usual position of the wires,
and to wind, in these coils, the primaries on the top; thus allowing the
use of a much larger primary, which, of course, reduces the danger of
overheating and increases the output of the coil. I make the primary on
each side at least one centimetre shorter than the secondary, to prevent
the breaking through on the ends, which would surely occur unless the
insulation on the top of the secondary be very thick, and this, of
course, would be disadvantageous.
When the primary is made movable, which is necessary in some
experiments, and many times convenient for the purposes of adjustment, I
cover the secondary with wax, and turn it off in a lathe to a diameter
slightly smaller than the inside of the primary coil. The latter I
provide with a handle reaching out of the oil, which serves to shift it
in any position along the secondary.
I will now venture to make, in regard to the general manipulation of
induction coils, a few observations bearing upon points which have not
been fully appreciated in earlier experiments with such coils, and are
even now often overlooked.
The secondary of the coil possesses usually such a high self-induction
that the current through the wire is inappreciable, and may be so even
when the terminals are joined by a conductor of small resistance. If
capacity is added to the terminals, the self-induction is counteracted,
and a stronger current is made to flow through the secondary, though its
terminals are insulated from each other. To one entirely unacquainted
with the properties of alternating currents nothing will look more
puzzling. This feature was illustrated in the experiment performed at
the beginning with the top plates of wire gauze attached to the
terminals and the rubber plate. When the plates of wire gauze were close
together, and a small arc passed between them, the arc _prevented_ a
strong current from passing through the secondary, because it did away
with the capacity on the terminals; when the rubber plate was inserted
between, the capacity of the condenser formed counteracted the
self-induction of the secondary, a stronger current passed now, the coil
performed more work, and the discharge was by far more powerful.
The first thing, then, in operating the induction coil is to combine
capacity with the secondary to overcome the self-induction. If the
frequencies and potentials are very high, gaseous matter should be
carefully kept away from the charged surfaces. If Leyden jars are used,
they should be immersed in oil, as otherwise considerable dissipation
may occur if the jars are greatly strained. When high frequencies are
used, it is of equal importance to combine a condenser with the primary.
One may use a condenser connected to the ends of the primary or to the
terminals of the alternator, but the latter is not to be recommended, as
the machine might be injured. The best way is undoubtedly to use the
condenser in series with the primary and with the alternator, and to
adjust its capacity so as to annul the self-induction of both the
latter. The condenser should be adjustable by very small steps, and for
a finer adjustment a small oil condenser with movable plates may be used
conveniently.
I think it best at this juncture to bring before you a phenomenon,
observed by me some time ago, which to the purely scientific
investigator may perhaps appear more interesting than any of the results
which I have the privilege to present to you this evening.
It may be quite properly ranked among the brush phenomena--in fact, it
is a brush, formed at, or near, a single terminal in high vacuum.
[Illustration: FIG. 141.]
[Illustration: FIG. 142.]
In bulbs provided with a conducting terminal, though it be of aluminum,
the brush has but an ephemeral existence, and cannot, unfortunately, be
indefinitely preserved in its most sensitive state, even in a bulb
devoid of any conducting electrode. In studying the phenomenon, by all
means a bulb having no leading-in wire should be used. I have found it
best to use bulbs constructed as indicated in Figs. 141 and 142.
In Fig. 141 the bulb comprises an incandescent lamp globe _L_, in the
neck of which is sealed a barometer tube _b_, the end of which is blown
out to form a small sphere _s_. This sphere should be sealed as closely
as possible in the centre of the large globe. Before sealing, a thin
tube _t_, of aluminum sheet, may be slipped in the barometer tube, but
it is not important to employ it.
The small hollow sphere _s_ is filled with some conducting powder, and a
wire _w_ is cemented in the neck for the purpose of connecting the
conducting powder with the generator.
The construction shown in Fig. 142 was chosen in order to remove from
the brush any conducting body which might possibly affect it. The bulb
consists in this case of a lamp globe _L_, which has a neck _n_,
provided with a tube _b_ and small sphere _s_, sealed to it, so that two
entirely independent compartments are formed, as indicated in the
drawing. When the bulb is in use the neck _n_ is provided with a tinfoil
coating, which is connected to the generator and acts inductively upon
the moderately rarefied and highly conducted gas inclosed in the neck.
From there the current passes through the tube _b_ into the small sphere
_s_, to act by induction upon the gas contained in the globe _L_.
It is of advantage to make the tube _t_ very thick, the hole through it
very small, and to blow the sphere _s_ very thin. It is of the greatest
importance that the sphere _s_ be placed in the centre of the globe _L_.
[Illustration: FIG. 143.]
Figs. 143, 144 and 145 indicate different forms, or stages, of the
brush. Fig. 143 shows the brush as it first appears in a bulb provided
with a conducting terminal; but, as in such a bulb it very soon
disappears--often after a few minutes--I will confine myself to the
description of the phenomenon as seen in a bulb without conducting
electrode. It is observed under the following conditions:
When the globe _L_ (Figs. 141 and 142) is exhausted to a very high
degree, generally the bulb is not excited upon connecting the wire _w_
(Fig. 141) or the tinfoil coating of the bulb (Fig. 142) to the
terminal of the induction coil. To excite it, it is usually sufficient
to grasp the globe _L_ with the hand. An intense phosphorescence then
spreads at first over the globe, but soon gives place to a white, misty
light. Shortly afterward one may notice that the luminosity is unevenly
distributed in the globe, and after passing the current for some time
the bulb appears as in Fig. 144. From this stage the phenomenon will
gradually pass to that indicated in Fig. 145, after some minutes, hours,
days or weeks, according as the bulb is worked. Warming the bulb or
increasing the potential hastens the transit.
[Illustration: FIG. 144.]
[Illustration: FIG. 145.]
When the brush assumes the form indicated in Fig. 145, it may be brought
to a state of extreme sensitiveness to electrostatic and magnetic
influence. The bulb hanging straight down from a wire, and all objects
being remote from it, the approach of the observer at a few paces from
the bulb will cause the brush to fly to the opposite side, and if he
walks around the bulb it will always keep on the opposite side. It may
begin to spin around the terminal long before it reaches that sensitive
stage. When it begins to turn around, principally, but also before, it
is affected by a magnet, and at a certain stage it is susceptible to
magnetic influence to an astonishing degree. A small permanent magnet,
with its poles at a distance of no more than two centimetres, will
affect it visibly at a distance of two metres, slowing down or
accelerating the rotation according to how it is held relatively to the
brush. I think I have observed that at the stage when it is most
sensitive to magnetic, it is not most sensitive to electrostatic,
influence. My explanation is, that the electrostatic attraction between
the brush and the glass of the bulb, which retards the rotation, grows
much quicker than the magnetic influence when the intensity of the
stream is increased.
When the bulb hangs with the globe _L_ down, the rotation is always
clockwise. In the southern hemisphere it would occur in the opposite
direction and on the equator the brush should not turn at all. The
rotation may be reversed by a magnet kept at some distance. The brush
rotates best, seemingly, when it is at right angles to the lines of
force of the earth. It very likely rotates, when at its maximum speed,
in synchronism with the alternations, say, 10,000 times a second. The
rotation can be slowed down or accelerated by the approach or receding
of the observer, or any conducting body, but it cannot be reversed by
putting the bulb in any position. When it is in the state of the highest
sensitiveness and the potential or frequency be varied, the
sensitiveness is rapidly diminished. Changing either of these but little
will generally stop the rotation. The sensitiveness is likewise affected
by the variations of temperature. To attain great sensitiveness it is
necessary to have the small sphere _s_ in the centre of the globe _L_,
as otherwise the electrostatic action of the glass of the globe will
tend to stop the rotation. The sphere _s_ should be small and of uniform
thickness; any dissymmetry of course has the effect to diminish the
sensitiveness.
The fact that the brush rotates in a definite direction in a permanent
magnetic field seems to show that in alternating currents of very high
frequency the positive and negative impulses are not equal, but that one
always preponderates over the other.
Of course, this rotation in one direction may be due to the action of
the two elements of the same current upon each other, or to the action
of the field produced by one of the elements upon the other, as in a
series motor, without necessarily one impulse being stronger than the
other. The fact that the brush turns, as far as I could observe, in any
position, would speak for this view. In such case it would turn at any
point of the earth's surface. But, on the other hand, it is then hard to
explain why a permanent magnet should reverse the rotation, and one must
assume the preponderance of impulses of one kind.
As to the causes of the formation of the brush or stream, I think it is
due to the electrostatic action of the globe and the dissymmetry of the
parts. If the small bulb _s_ and the globe _L_ were perfect concentric
spheres, and the glass throughout of the same thickness and quality, I
think the brush would not form, as the tendency to pass would be equal
on all sides. That the formation of the stream is due to an irregularity
is apparent from the fact that it has the tendency to remain in one
position, and rotation occurs most generally only when it is brought out
of this position by electrostatic or magnetic influence. When in an
extremely sensitive state it rests in one position, most curious
experiments may be performed with it. For instance, the experimenter
may, by selecting a proper position, approach the hand at a certain
considerable distance to the bulb, and he may cause the brush to pass
off by merely stiffening the muscles of the arm. When it begins to
rotate slowly, and the hands are held at a proper distance, it is
impossible to make even the slightest motion without producing a visible
effect upon the brush. A metal plate connected to the other terminal of
the coil affects it at a great distance, slowing down the rotation often
to one turn a second.
I am firmly convinced that such a brush, when we learn how to produce it
properly, will prove a valuable aid in the investigation of the nature
of the forces acting in an electrostatic or magnetic field. If there is
any motion which is measurable going on in the space, such a brush ought
to reveal it. It is, so to speak, a beam of light, frictionless, devoid
of inertia.
I think that it may find practical applications in telegraphy. With such
a brush it would be possible to send dispatches across the Atlantic, for
instance, with any speed, since its sensitiveness may be so great that
the slightest changes will affect it. If it were possible to make the
stream more intense and very narrow, its deflections could be easily
photographed.
I have been interested to find whether there is a rotation of the stream
itself, or whether there is simply a stress traveling around the bulb.
For this purpose I mounted a light mica fan so that its vanes were in
the path of the brush. If the stream itself was rotating the fan would
be spun around. I could produce no distinct rotation of the fan,
although I tried the experiment repeatedly; but as the fan exerted a
noticeable influence on the stream, and the apparent rotation of the
latter was, in this case, never quite satisfactory, the experiment did
not appear to be conclusive.
I have been unable to produce the phenomenon with the disruptive
discharge coil, although every other of these phenomena can be well
produced by it--many, in fact, much better than with coils operated from
an alternator.
It may be possible to produce the brush by impulses of one direction, or
even by a steady potential, in which case it would be still more
sensitive to magnetic influence.
In operating an induction coil with rapidly alternating currents, we
realize with astonishment, for the first time, the great importance of
the relation of capacity, self-induction and frequency as regards the
general results. The effects of capacity are the most striking, for in
these experiments, since the self-induction and frequency both are high,
the critical capacity is very small, and need be but slightly varied to
produce a very considerable change. The experimenter may bring his body
in contact with the terminals of the secondary of the coil, or attach to
one or both terminals insulated bodies of very small bulk, such as
bulbs, and he may produce a considerable rise or fall of potential, and
greatly affect the flow of the current through the primary. In the
experiment before shown, in which a brush appears at a wire attached to
one terminal, and the wire is vibrated when the experimenter brings his
insulated body in contact with the other terminal of the coil, the
sudden rise of potential was made evident.
I may show you the behavior of the coil in another manner which
possesses a feature of some interest. I have here a little light fan of
aluminum sheet, fastened to a needle and arranged to rotate freely in a
metal piece screwed to one of the terminals of the coil. When the coil
is set to work, the molecules of the air are rhythmically attracted and
repelled. As the force with which they are repelled is greater than that
with which they are attracted, it results that there is a repulsion
exerted on the surfaces of the fan. If the fan were made simply of a
metal sheet, the repulsion would be equal on the opposite sides, and
would produce no effect. But if one of the opposing surfaces is
screened, or if, generally speaking, the bombardment on this side is
weakened in some way or other, there remains the repulsion exerted upon
the other, and the fan is set in rotation. The screening is best
effected by fastening upon one of the opposing sides of the fan
insulated conducting coatings, or, if the fan is made in the shape of an
ordinary propeller screw, by fastening on one side, and close to it, an
insulated metal plate. The static screen may, however, be omitted, and
simply a thickness of insulating material fastened to one of the sides
of the fan.
To show the behavior of the coil, the fan may be placed upon the
terminal and it will readily rotate when the coil is operated by
currents of very high frequency. With a steady potential, of course, and
even with alternating currents of very low frequency, it would not turn,
because of the very slow exchange of air and, consequently, smaller
bombardment; but in the latter case it might turn if the potential were
excessive. With a pin wheel, quite the opposite rule holds good; it
rotates best with a steady potential, and the effort is the smaller the
higher the frequency. Now, it is very easy to adjust the conditions so
that the potential is normally not sufficient to turn the fan, but that
by connecting the other terminal of the coil with an insulated body it
rises to a much greater value, so as to rotate the fan, and it is
likewise possible to stop the rotation by connecting to the terminal a
body of different size, thereby diminishing the potential.
Instead of using the fan in this experiment, we may use the "electric"
radiometer with similar effect. But in this case it will be found that
the vanes will rotate only at high exhaustion or at ordinary pressures;
they will not rotate at moderate pressures, when the air is highly
conducting. This curious observation was made conjointly by Professor
Crookes and myself. I attribute the result to the high conductivity of
the air, the molecules of which then do not act as independent carriers
of electric charges, but act all together as a single conducting body.
In such case, of course, if there is any repulsion at all of the
molecules from the vanes, it must be very small. It is possible,
however, that the result is in part due to the fact that the greater
part of the discharge passes from the leading-in wire through the highly
conducting gas, instead of passing off from the conducting vanes.
In trying the preceding experiment with the electric radiometer the
potential should not exceed a certain limit, as then the electrostatic
attraction between the vanes and the glass of the bulb may be so great
as to stop the rotation.
A most curious feature of alternate currents of high frequencies and
potentials is that they enable us to perform many experiments by the use
of one wire only. In many respects this feature is of great interest.
In a type of alternate current motor invented by me some years ago I
produced rotation by inducing, by means of a single alternating current
passed through a motor circuit, in the mass or other circuits of the
motor, secondary currents, which, jointly with the primary or inducing
current, created a moving field of force. A simple but crude form of
such a motor is obtained by winding upon an iron core a primary, and
close to it a secondary coil, joining the ends of the latter and placing
a freely movable metal disc within the influence of the field produced
by both. The iron core is employed for obvious reasons, but it is not
essential to the operation. To improve the motor, the iron core is made
to encircle the armature. Again to improve, the secondary coil is made
to partly overlap the primary, so that it cannot free itself from a
strong inductive action of the latter, repel its lines as it may. Once
more to improve, the proper difference of phase is obtained between the
primary and secondary currents by a condenser, self-induction,
resistance or equivalent windings.
I had discovered, however, that rotation is produced by means of a
single coil and core; my explanation of the phenomenon, and leading
thought in trying the experiment, being that there must be a true time
lag in the magnetization of the core. I remember the pleasure I had
when, in the writings of Professor Ayrton, which came later to my hand,
I found the idea of the time lag advocated. Whether there is a true time
lag, or whether the retardation is due to eddy currents circulating in
minute paths, must remain an open question, but the fact is that a coil
wound upon an iron core and traversed by an alternating current creates
a moving field of force, capable of setting an armature in rotation. It
is of some interest, in conjunction with the historical Arago
experiment, to mention that in lag or phase motors I have produced
rotation in the opposite direction to the moving field, which means that
in that experiment the magnet may not rotate, or may even rotate in the
opposite direction to the moving disc. Here, then, is a motor
(diagrammatically illustrated in Fig. 146), comprising a coil and iron
core, and a freely movable copper disc in proximity to the latter.
[Illustration: FIG. 146.]
To demonstrate a novel and interesting feature, I have, for a reason
which I will explain, selected this type of motor. When the ends of the
coil are connected to the terminals of an alternator the disc is set in
rotation. But it is not this experiment, now well known, which I desire
to perform. What I wish to show you is that this motor rotates with
_one single_ connection between it and the generator; that is to say,
one terminal of the motor is connected to one terminal of the
generator--in this case the secondary of a high-tension induction
coil--the other terminals of motor and generator being insulated in
space. To produce rotation it is generally (but not absolutely)
necessary to connect the free end of the motor coil to an insulated body
of some size. The experimenter's body is more than sufficient. If he
touches the free terminal with an object held in the hand, a current
passes through the coil and the copper disc is set in rotation. If an
exhausted tube is put in series with the coil, the tube lights
brilliantly, showing the passage of a strong current. Instead of the
experimenter's body, a small metal sheet suspended on a cord may be used
with the same result. In this case the plate acts as a condenser in
series with the coil. It counteracts the self-induction of the latter
and allows a strong current to pass. In such a combination, the greater
the self-induction of the coil the smaller need be the plate, and this
means that a lower frequency, or eventually a lower potential, is
required to operate the motor. A single coil wound upon a core has a
high self-induction; for this reason, principally, this type of motor
was chosen to perform the experiment. Were a secondary closed coil wound
upon the core, it would tend to diminish the self-induction, and then
it would be necessary to employ a much higher frequency and potential.
Neither would be advisable, for a higher potential would endanger the
insulation of the small primary coil, and a higher frequency would
result in a materially diminished torque.
It should be remarked that when such a motor with a closed secondary is
used, it is not at all easy to obtain rotation with excessive
frequencies, as the secondary cuts off almost completely the lines of
the primary--and this, of course, the more, the higher the
frequency--and allows the passage of but a minute current. In such a
case, unless the secondary is closed through a condenser, it is almost
essential, in order to produce rotation, to make the primary and
secondary coils overlap each other more or less.
But there is an additional feature of interest about this motor, namely,
it is not necessary to have even a single connection between the motor
and generator, except, perhaps, through the ground; for not only is an
insulated plate capable of giving off energy into space, but it is
likewise capable of deriving it from an alternating electrostatic field,
though in the latter case the available energy is much smaller. In this
instance one of the motor terminals is connected to the insulated plate
or body located within the alternating electrostatic field, and the
other terminal preferably to the ground.
It is quite possible, however, that such "no wire" motors, as they might
be called, could be operated by conduction through the rarefied air at
considerable distances. Alternate currents, especially of high
frequencies, pass with astonishing freedom through even slightly
rarefied gases. The upper strata of the air are rarefied. To reach a
number of miles out into space requires the overcoming of difficulties
of a merely mechanical nature. There is no doubt that with the enormous
potentials obtainable by the use of high frequencies and oil insulation,
luminous discharges might be passed through many miles of rarefied air,
and that, by thus directing the energy of many hundreds or thousands of
horse-power, motors or lamps might be operated at considerable distances
from stationary sources. But such schemes are mentioned merely as
possibilities. We shall have no need to transmit power in this way. We
shall have no need to _transmit_ power at all. Ere many generations
pass, our machinery will be driven by a power obtainable at any point of
the universe. This idea is not novel. Men have been led to it long ago
by instinct or reason. It has been expressed in many ways, and in many
places, in the history of old and new. We find it in the delightful myth
of Antheus, who derives power from the earth; we find it among the
subtle speculations of one of your splendid mathematicians, and in many
hints and statements of thinkers of the present time. Throughout space
there is energy. Is this energy static or kinetic? If static our hopes
are in vain; if kinetic--and this we know it is, for certain--then it is
a mere question of time when men will succeed in attaching their
machinery to the very wheelwork of nature. Of all, living or dead,
Crookes came nearest to doing it. His radiometer will turn in the light
of day and in the darkness of the night; it will turn everywhere where
there is heat, and heat is everywhere. But, unfortunately, this
beautiful little machine, while it goes down to posterity as the most
interesting, must likewise be put on record as the most inefficient
machine ever invented!
The preceding experiment is only one of many equally interesting
experiments which may be performed by the use of only one wire with
alternations of high potential and frequency. We may connect an
insulated line to a source of such currents, we may pass an
inappreciable current over the line, and on any point of the same we are
able to obtain a heavy current, capable of fusing a thick copper wire.
Or we may, by the help of some artifice, decompose a solution in any
electrolytic cell by connecting only one pole of the cell to the line or
source of energy. Or we may, by attaching to the line, or only bringing
into its vicinity, light up an incandescent lamp, an exhausted tube, or
a phosphorescent bulb.
However impracticable this plan of working may appear in many cases, it
certainly seems practicable, and even recommendable, in the production
of light. A perfected lamp would require but little energy, and if wires
were used at all we ought to be able to supply that energy without a
return wire.
It is now a fact that a body may be rendered incandescent or
phosphorescent by bringing it either in single contact or merely in the
vicinity of a source of electric impulses of the proper character, and
that in this manner a quantity of light sufficient to afford a practical
illuminant may be produced. It is, therefore, to say the least, worth
while to attempt to determine the best conditions and to invent the best
appliances for attaining this object.
Some experiences have already been gained in this direction, and I will
dwell on them briefly, in the hope that they might prove useful.
The heating of a conducting body inclosed in a bulb, and connected to a
source of rapidly alternating electric impulses, is dependent on so many
things of a different nature, that it would be difficult to give a
generally applicable rule under which the maximum heating occurs. As
regards the size of the vessel, I have lately found that at ordinary or
only slightly differing atmospheric pressures, when air is a good
insulator, and hence practically the same amount of energy by a certain
potential and frequency is given off from the body, whether the bulb be
small or large, the body is brought to a higher temperature if enclosed
in a small bulb, because of the better confinement of heat in this case.
At lower pressures, when air becomes more or less conducting, or if the
air be sufficiently warmed to become conducting, the body is rendered
more intensely incandescent in a large bulb, obviously because, under
otherwise equal conditions of test, more energy may be given off from
the body when the bulb is large.
At very high degrees of exhaustion, when the matter in the bulb becomes
"radiant," a large bulb has still an advantage, but a comparatively
slight one, over the small bulb.
Finally, at excessively high degrees of exhaustion, which cannot be
reached except by the employment of special means, there seems to be,
beyond a certain and rather small size of vessel, no perceptible
difference in the heating.
These observations were the result of a number of experiments, of which
one, showing the effect of the size of the bulb at a high degree of
exhaustion, may be described and shown here, as it presents a feature of
interest. Three spherical bulbs of 2 inches, 3 inches and 4 inches
diameter were taken, and in the centre of each was mounted an equal
length of an ordinary incandescent lamp filament of uniform thickness.
In each bulb the piece of filament was fastened to the leading-in wire
of platinum, contained in a glass stem sealed in the bulb; care being
taken, of course, to make everything as nearly alike as possible. On
each glass stem in the inside of the bulb was slipped a highly polished
tube made of aluminum sheet, which fitted the stem and was held on it by
spring pressure. The function of this aluminum tube will be explained
subsequently. In each bulb an equal length of filament protruded above
the metal tube. It is sufficient to say now that under these conditions
equal lengths of filament of the same thickness--in other words, bodies
of equal bulk--were brought to incandescence. The three bulbs were
sealed to a glass tube, which was connected to a Sprengel pump. When a
high vacuum had been reached, the glass tube carrying the bulbs was
sealed off. A current was then turned on successively on each bulb, and
it was found that the filaments came to about the same brightness, and,
if anything, the smallest bulb, which was placed midway between the two
larger ones, may have been slightly brighter. This result was expected,
for when either of the bulbs was connected to the coil the luminosity
spread through the other two, hence the three bulbs constituted really
one vessel. When all the three bulbs were connected in multiple arc to
the coil, in the largest of them the filament glowed brightest, in the
next smaller it was a little less bright, and in the smallest it only
came to redness. The bulbs were then sealed off and separately tried.
The brightness of the filaments was now such as would have been expected
on the supposition that the energy given off was proportionate to the
surface of the bulb, this surface in each case representing one of the
coatings of a condenser. Accordingly, there was less difference between
the largest and the middle sized than between the latter and the
smallest bulb.
An interesting observation was made in this experiment. The three bulbs
were suspended from a straight bare wire connected to a terminal of a
coil, the largest bulb being placed at the end of the wire, at some
distance from it the smallest bulb, and at an equal distance from the
latter the middle-sized one. The carbons glowed then in both the larger
bulbs about as expected, but the smallest did not get its share by far.
This observation led me to exchange the position of the bulbs, and I
then observed that whichever of the bulbs was in the middle was by far
less bright than it was in any other position. This mystifying result
was, of course, found to be due to the electrostatic action between the
bulbs. When they were placed at a considerable distance, or when they
were attached to the corners of an equilateral triangle of copper wire,
they glowed in about the order determined by their surfaces.
As to the shape of the vessel, it is also of some importance, especially
at high degrees of exhaustion. Of all the possible constructions, it
seems that a spherical globe with the refractory body mounted in its
centre is the best to employ. By experience it has been demonstrated
that in such a globe a refractory body of a given bulk is more easily
brought to incandescence than when differently shaped bulbs are used.
There is also an advantage in giving to the incandescent body the shape
of a sphere, for self-evident reasons. In any case the body should be
mounted in the centre, where the atoms rebounding from the glass
collide. This object is best attained in the spherical bulb; but it is
also attained in a cylindrical vessel with one or two straight filaments
coinciding with its axis, and possibly also in parabolical or spherical
bulbs with refractory body or bodies placed in the focus or foci of the
same; though the latter is not probable, as the electrified atoms should
in all cases rebound normally from the surface they strike, unless the
speed were excessive, in which case they _would_ probably follow the
general law of reflection. No matter what shape the vessel may have, if
the exhaustion be low, a filament mounted in the globe is brought to the
same degree of incandescence in all parts; but if the exhaustion be high
and the bulb be spherical or pear-shaped, as usual, focal points form
and the filament is heated to a higher degree at or near such points.
To illustrate the effect, I have here two small bulbs which are alike,
only one is exhausted to a low and the other to a very high degree. When
connected to the coil, the filament in the former glows uniformly
throughout all its length; whereas in the latter, that portion of the
filament which is in the centre of the bulb glows far more intensely
than the rest. A curious point is that the phenomenon occurs even if two
filaments are mounted in a bulb, each being connected to one terminal of
the coil, and, what is still more curious, if they be very near
together, provided the vacuum be very high. I noted in experiments with
such bulbs that the filaments would give way usually at a certain point,
and in the first trials I attributed it to a defect in the carbon. But
when the phenomenon occurred many times in succession I recognized its
real cause.
In order to bring a refractory body inclosed in a bulb to incandescence,
it is desirable, on account of economy, that all the energy supplied to
the bulb from the source should reach without loss the body to be
heated; from there, and from nowhere else, it should be radiated. It is,
of course, out of the question to reach this theoretical result, but it
is possible by a proper construction of the illuminating device to
approximate it more or less.
For many reasons, the refractory body is placed in the centre of the
bulb, and it is usually supported on a glass stem containing the
leading-in wire. As the potential of this wire is alternated, the
rarefied gas surrounding the stem is acted upon inductively, and the
glass stem is violently bombarded and heated. In this manner by far the
greater portion of the energy supplied to the bulb--especially when
exceedingly high frequencies are used--may be lost for the purpose
contemplated. To obviate this loss, or at least to reduce it to a
minimum, I usually screen the rarefied gas surrounding the stem from the
inductive action of the leading-in wire by providing the stem with a
tube or coating of conducting material. It seems beyond doubt that the
best among metals to employ for this purpose is aluminum, on account of
its many remarkable properties. Its only fault is that it is easily
fusible, and, therefore, its distance from the incandescing body should
be properly estimated. Usually, a thin tube, of a diameter somewhat
smaller than that of the glass stem, is made of the finest aluminum
sheet, and slipped on the stem. The tube is conveniently prepared by
wrapping around a rod fastened in a lathe a piece of aluminum sheet of
proper size, grasping the sheet firmly with clean chamois leather or
blotting paper, and spinning the rod very fast. The sheet is wound
tightly around the rod, and a highly polished tube of one or three
layers of the sheet is obtained. When slipped on the stem, the pressure
is generally sufficient to prevent it from slipping off, but, for
safety, the lower edge of the sheet may be turned inside. The upper
inside corner of the sheet--that is, the one which is nearest to the
refractory incandescent body--should be cut out diagonally, as it often
happens that, in consequence of the intense heat, this corner turns
toward the inside and comes very near to, or in contact with, the wire,
or filament, supporting the refractory body. The greater part of the
energy supplied to the bulb is then used up in heating the metal tube,
and the bulb is rendered useless for the purpose. The aluminum sheet
should project above the glass stem more or less--one inch or so--or
else, if the glass be too close to the incandescing body, it may be
strongly heated and become more or less conducting, whereupon it may be
ruptured, or may, by its conductivity, establish a good electrical
connection between the metal tube and the leading-in wire, in which
case, again, most of the energy will be lost in heating the former.
Perhaps the best way is to make the top of the glass tube, for about an
inch, of a much smaller diameter. To still further reduce the danger
arising from the heating of the glass stem, and also with the view of
preventing an electrical connection between the metal tube and the
electrode, I preferably wrap the stem with several layers of thin mica,
which extends at least as far as the metal tube. In some bulbs I have
also used an outside insulating cover.
The preceding remarks are only made to aid the experimenter in the first
trials, for the difficulties which he encounters he may soon find means
to overcome in his own way.
To illustrate the effect of the screen, and the advantage of using it, I
have here two bulbs of the same size, with their stems, leading-in wires
and incandescent lamp filaments tied to the latter, as nearly alike as
possible. The stem of one bulb is provided with an aluminum tube, the
stem of the other has none. Originally the two bulbs were joined by a
tube which was connected to a Sprengel pump. When a high vacuum had been
reached, first the connecting tube, and then the bulbs, were sealed off;
they are therefore of the same degree of exhaustion. When they are
separately connected to the coil giving a certain potential, the carbon
filament in the bulb provided with the aluminum screen is rendered
highly incandescent, while the filament in the other bulb may, with the
same potential, not even come to redness, although in reality the latter
bulb takes generally more energy than the former. When they are both
connected together to the terminal, the difference is even more
apparent, showing the importance of the screening. The metal tube placed
on the stem containing the leading-in wire performs really two distinct
functions: First, it acts more or less as an electrostatic screen, thus
economizing the energy supplied to the bulb; and, second, to whatever
extent it may fail to act electrostatically, it acts mechanically,
preventing the bombardment, and consequently intense heating and
possible deterioration of the slender support of the refractory
incandescent body, or of the glass stem containing the leading-in wire.
I say _slender_ support, for it is evident that in order to confine the
heat more completely to the incandescing body its support should be very
thin, so as to carry away the smallest possible amount of heat by
conduction. Of all the supports used I have found an ordinary
incandescent lamp filament to be the best, principally because among
conductors it can withstand the highest degree of heat.
The effectiveness of the metal tube as an electrostatic screen depends
largely on the degree of exhaustion.
At excessively high degrees of exhaustion--which are reached by using
great care and special means in connection with the Sprengel pump--when
the matter in the globe is in the ultra-radiant state, it acts most
perfectly. The shadow of the upper edge of the tube is then sharply
defined upon the bulb.
At a somewhat lower degree of exhaustion, which is about the ordinary
"non-striking" vacuum, and generally as long as the matter moves
predominantly in straight lines, the screen still does well. In
elucidation of the preceding remark it is necessary to state that what
is a "non-striking" vacuum for a coil operated as ordinarily, by
impulses, or currents, of low frequency, is not so, by far, when the
coil is operated by currents of very high frequency. In such case the
discharge may pass with great freedom through the rarefied gas through
which a low frequency discharge may not pass, even though the potential
be much higher. At ordinary atmospheric pressures just the reverse rule
holds good: the higher the frequency, the less the spark discharge is
able to jump between the terminals, especially if they are knobs or
spheres of some size.
Finally, at very low degrees of exhaustion, when the gas is well
conducting, the metal tube not only does not act as an electrostatic
screen, but even is a drawback, aiding to a considerable extent the
dissipation of the energy laterally from the leading-in wire. This, of
course, is to be expected. In this case, namely, the metal tube is in
good electrical connection with the leading-in wire, and most of the
bombardment is directed upon the tube. As long as the electrical
connection is not good, the conducting tube is always of some advantage,
for although it may not greatly economize energy, still it protects the
support of the refractory button, and is the means of concentrating more
energy upon the same.
To whatever extent the aluminum tube performs the function of a screen,
its usefulness is therefore limited to very high degrees of exhaustion
when it is insulated from the electrode--that is, when the gas as a
whole is non-conducting, and the molecules, or atoms, act as independent
carriers of electric charges.
In addition to acting as a more or less effective screen, in the true
meaning of the word, the conducting tube or coating may also act, by
reason of its conductivity, as a sort of equalizer or dampener of the
bombardment against the stem. To be explicit, I assume the action to be
as follows: Suppose a rhythmical bombardment to occur against the
conducting tube by reason of its imperfect action as a screen, it
certainly must happen that some molecules, or atoms, strike the tube
sooner than others. Those which come first in contact with it give up
their superfluous charge, and the tube is electrified, the
electrification instantly spreading over its surface. But this must
diminish the energy lost in the bombardment, for two reasons: first, the
charge given up by the atoms spreads over a great area, and hence the
electric density at any point is small, and the atoms are repelled with
less energy than they would be if they struck against a good insulator;
secondly, as the tube is electrified by the atoms which first come in
contact with it, the progress of the following atoms against the tube is
more or less checked by the repulsion which the electrified tube must
exert upon the similarly electrified atoms. This repulsion may perhaps
be sufficient to prevent a large portion of the atoms from striking the
tube, but at any rate it must diminish the energy of their impact. It is
clear that when the exhaustion is very low, and the rarefied gas well
conducting, neither of the above effects can occur, and, on the other
hand, the fewer the atoms, with the greater freedom they move; in other
words, the higher the degree of exhaustion, up to a limit, the more
telling will be both the effects.
[Illustration: FIG. 147.]
[Illustration: FIG. 148.]
What I have just said may afford an explanation of the phenomenon
observed by Prof. Crookes, namely, that a discharge through a bulb is
established with much greater facility when an insulator than when a
conductor is present in the same. In my opinion, the conductor acts as a
dampener of the motion of the atoms in the two ways pointed out; hence,
to cause a visible discharge to pass through the bulb, a much higher
potential is needed if a conductor, especially of much surface, be
present.
For the sake of elucidating of some of the remarks before made, I must
now refer to Figs. 147, 148 and 149, which illustrate various
arrangements with a type of bulb most generally used.
Fig. 147 is a section through a spherical bulb L, with the glass stem
_s_, contains the leading-in wire _w_, which has a lamp filament _l_
fastened to it, serving to support the refractory button _m_ in the
centre. M is a sheet of thin mica wound in several layers around the
stem _s_, and _a_ is the aluminum tube.
Fig. 148 illustrates such a bulb in a somewhat more advanced stage of
perfection. A metallic tube S is fastened by means of some cement to the
neck of the tube. In the tube is screwed a plug P, of insulating
material, in the centre of which is fastened a metallic terminal _t_,
for the connection to the leading-in wire _w_. This terminal must be
well insulated from the metal tube S; therefore, if the cement used is
conducting--and most generally it is sufficiently so--the space between
the plug P and the neck of the bulb should be filled with some good
insulating material, such as mica powder.
Fig. 149 shows a bulb made for experimental purposes. In this bulb the
aluminum tube is provided with an external connection, which serves to
investigate the effect of the tube under various conditions. It is
referred to chiefly to suggest a line of experiment followed.
Since the bombardment against the stem containing the leading-in wire is
due to the inductive action of the latter upon the rarefied gas, it is
of advantage to reduce this action as far as practicable by employing a
very thin wire, surrounded by a very thick insulation of glass or other
material, and by making the wire passing through the rarefied gas as
short as practicable. To combine these features I employ a large tube T
(Fig. 150), which protrudes into the bulb to some distance, and carries
on the top a very short glass stem _s_, into which is sealed the
leading-in wire _w_, and I protect the top of the glass stem against the
heat by a small aluminum tube _a_ and a layer of mica underneath the
same, as usual. The wire _w_, passing through the large tube to the
outside of the bulb, should be well insulated--with a glass tube, for
instance--and the space between ought to be filled out with some
excellent insulator. Among many insulating powders I have found that
mica powder is the best to employ. If this precaution is not taken, the
tube T, protruding into the bulb, will surely be cracked in consequence
of the heating by the brushes which are apt to form in the upper part of
the tube, near the exhausted globe, especially if the vacuum be
excellent, and therefore the potential necessary to operate the lamp be
very high.
[Illustration: FIG. 149.]
[Illustration: FIG. 150.]
Fig. 151 illustrates a similar arrangement, with a large tube T
protruding into the part of the bulb containing the refractory button
_m_. In this case the wire leading from the outside into the bulb is
omitted, the energy required being supplied through condenser coatings C
C. The insulating packing P should in this construction be tightly
fitting to the glass, and rather wide, or otherwise the discharge might
avoid passing through the wire _w_, which connects the inside condenser
coating to the incandescent button _m_.
The molecular bombardment against the glass stem in the bulb is a source
of great trouble. As an illustration I will cite a phenomenon only too
frequently and unwillingly observed. A bulb, preferably a large one, may
be taken, and a good conducting body, such as a piece of carbon, may be
mounted in it upon a platinum wire sealed in the glass stem. The bulb
may be exhausted to a fairly high degree, nearly to the point when
phosphorescence begins to appear. When the bulb is connected with the
coil, the piece of carbon, if small, may become highly incandescent at
first, but its brightness immediately diminishes, and then the discharge
may break through the glass somewhere in the middle of the stem, in the
form of bright sparks, in spite of the fact that the platinum wire is in
good electrical connection with the rarefied gas through the piece of
carbon or metal at the top. The first sparks are singularly bright,
recalling those drawn from a clear surface of mercury. But, as they heat
the glass rapidly, they, of course, lose their brightness, and cease
when the glass at the ruptured place becomes incandescent, or generally
sufficiently hot to conduct. When observed for the first time the
phenomenon must appear very curious, and shows in a striking manner how
radically different alternate currents, or impulses, of high frequency
behave, as compared with steady currents, or currents of low frequency.
With such currents--namely, the latter--the phenomenon would of course
not occur. When frequencies such as are obtained by mechanical means are
used, I think that the rupture of the glass is more or less the
consequence of the bombardment, which warms it up and impairs its
insulating power; but with frequencies obtainable with condensers I have
no doubt that the glass may give way without previous heating. Although
this appears most singular at first, it is in reality what we might
expect to occur. The energy supplied to the wire leading into the bulb
is given off partly by direct action through the carbon button, and
partly by inductive action through the glass surrounding the wire. The
case is thus analogous to that in which a condenser shunted by a
conductor of low resistance is connected to a source of alternating
current. As long as the frequencies are low, the conductor gets the most
and the condenser is perfectly safe; but when the frequency becomes
excessive, the _role_ of the conductor may become quite insignificant.
In the latter case the difference of potential at the terminals of the
condenser may become so great as to rupture the dielectric,
notwithstanding the fact that the terminals are joined by a conductor of
low resistance.
It is, of course, not necessary, when it is desired to produce the
incandescence of a body inclosed in a bulb by means of these currents,
that the body should be a conductor, for even a perfect non-conductor
may be quite as readily heated. For this purpose it is sufficient to
surround a conducting electrode with a non-conducting material, as, for
instance, in the bulb described before in Fig. 150, in which a thin
incandescent lamp filament is coated with a non-conductor, and supports
a button of the same material on the top. At the start the bombardment
goes on by inductive action through the non-conductor, until the same is
sufficiently heated to become conducting, when the bombardment continues
in the ordinary way.
[Illustration: FIG. 151.]
[Illustration: FIG. 152.]
A different arrangement used in some of the bulbs constructed is
illustrated in Fig. 152. In this instance a non-conductor _m_ is mounted
in a piece of common arc light carbon so as to project some small
distance above the latter. The carbon piece is connected to the
leading-in wire passing through a glass stem, which is wrapped with
several layers of mica. An aluminum tube _a_ is employed as usual for
screening. It is so arranged that it reaches very nearly as high as the
carbon and only the non-conductor _m_ projects a little above it. The
bombardment goes at first against the upper surface of carbon, the lower
parts being protected by the aluminum tube. As soon, however, as the
non-conductor _m_ is heated it is rendered good conducting, and then it
becomes the centre of the bombardment, being most exposed to the same.
I have also constructed during these experiments many such single-wire
bulbs with or without internal electrode, in which the radiant matter
was projected against, or focused upon, the body to be rendered
incandescent. Fig. 153 (page 263) illustrates one of the bulbs used. It
consists of a spherical globe L, provided with a long neck _n_, on top,
for increasing the action in some cases by the application of an
external conducting coating. The globe L is blown out on the bottom into
a very small bulb _b_, which serves to hold it firmly in a socket S of
insulating material into which it is cemented. A fine lamp filament _f_,
supported on a wire _w_, passes through the centre of the globe L. The
filament is rendered incandescent in the middle portion, where the
bombardment proceeding from the lower inside surface of the globe is
most intense. The lower portion of the globe, as far as the socket S
reaches, is rendered conducting, either by a tinfoil coating or
otherwise, and the external electrode is connected to a terminal of the
coil.
The arrangement diagrammatically indicated in Fig. 153 was found to be
an inferior one when it was desired to render incandescent a filament or
button supported in the centre of the globe, but it was convenient when
the object was to excite phosphorescence.
In many experiments in which bodies of different kind were mounted in
the bulb as, for instance, indicated in Fig. 152, some observations of
interest were made.
It was found, among other things, that in such cases, no matter where
the bombardment began, just as soon as a high temperature was reached
there was generally one of the bodies which seemed to take most of the
bombardment upon itself, the other, or others, being thereby relieved.
The quality appeared to depend principally on the point of fusion, and
on the facility with which the body was "evaporated," or, generally
speaking, disintegrated--meaning by the latter term not only the
throwing off of atoms, but likewise of large lumps. The observation made
was in accordance with generally accepted notions. In a highly exhausted
bulb, electricity is carried off from the electrode by independent
carriers, which are partly the atoms, or molecules, of the residual
atmosphere, and partly the atoms, molecules, or lumps thrown off from
the electrode. If the electrode is composed of bodies of different
character, and if one of these is more easily disintegrated than the
other, most of the electricity supplied is carried off from that body,
which is then brought to a higher temperature than the others, and this
the more, as upon an increase of the temperature the body is still more
easily disintegrated.
It seems to me quite probable that a similar process takes place in the
bulb even with a homogeneous electrode, and I think it to be the
principal cause of the disintegration. There is bound to be some
irregularity, even if the surface is highly polished, which, of course,
is impossible with most of the refractory bodies employed as electrodes.
Assume that a point of the electrode gets hotter; instantly most of the
discharge passes through that point, and a minute patch it probably
fused and evaporated. It is now possible that in consequence of the
violent disintegration the spot attacked sinks in temperature, or that a
counter force is created, as in an arc; at any rate, the local tearing
off meets with the limitations incident to the experiment, whereupon the
same process occurs on another place. To the eye the electrode appears
uniformly brilliant, but there are upon it points constantly shifting
and wandering around, of a temperature far above the mean, and this
materially hastens the process of deterioration. That some such thing
occurs, at least when the electrode is at a lower temperature,
sufficient experimental evidence can be obtained in the following
manner: Exhaust a bulb to a very high degree, so that with a fairly high
potential the discharge cannot pass--that is, not a _luminous_ one, for
a weak invisible discharge occurs always, in all probability. Now raise
slowly and carefully the potential, leaving the primary current on no
more than for an instant. At a certain point, two, three, or half a
dozen phosphorescent spots will appear on the globe. These places of the
glass are evidently more violently bombarded than others, this being due
to the unevenly distributed electric density, necessitated, of course,
by sharp projections, or, generally speaking, irregularities of the
electrode. But the luminous patches are constantly changing in position,
which is especially well observable if one manages to produce very few,
and this indicates that the configuration of the electrode is rapidly
changing.
From experiences of this kind I am led to infer that, in order to be
most durable, the refractory button in the bulb should be in the form of
a sphere with a highly polished surface. Such a small sphere could be
manufactured from a diamond or some other crystal, but a better way
would be to fuse, by the employment of extreme degrees of temperature,
some oxide--as, for instance, zirconia--into a small drop, and then keep
it in the bulb at a temperature somewhat below its point of fusion.
Interesting and useful results can, no doubt, be reached in the
direction of extreme degrees of heat. How can such high temperatures be
arrived at? How are the highest degrees of heat reached in nature? By
the impact of stars, by high speeds and collisions. In a collision any
rate of heat generation may be attained. In a chemical process we are
limited. When oxygen and hydrogen combine, they fall, metaphorically
speaking, from a definite height. We cannot go very far with a blast,
nor by confining heat in a furnace, but in an exhausted bulb we can
concentrate any amount of energy upon a minute button. Leaving
practicability out of consideration, this, then, would be the means
which, in my opinion, would enable us to reach the highest temperature.
But a great difficulty when proceeding in this way is encountered,
namely, in most cases the body is carried off before it can fuse and
form a drop. This difficulty exists principally with an oxide, such as
zirconia, because it cannot be compressed in so hard a cake that it
would not be carried off quickly. I have endeavored repeatedly to fuse
zirconia, placing it in a cup of arc light carbon, as indicated in Fig.
152. It glowed with a most intense light, and the stream of the
particles projected out of the carbon cup was of a vivid white; but
whether it was compressed in a cake or made into a paste with carbon, it
was carried off before it could be fused. The carbon cup, containing
zirconia, had to be mounted very low in the neck of a large bulb, as the
heating of the glass by the projected particles of the oxide was so
rapid that in the first trial the bulb was cracked almost in an instant,
when the current was turned on. The heating of the glass by the
projected particles was found to be always greater when the carbon cup
contained a body which was rapidly carried off--I presume, because in
such cases, with the same potential, higher speeds were reached, and
also because, per unit of time, more matter was projected--that is, more
particles would strike the glass.
The before-mentioned difficulty did not exist, however, when the body
mounted in the carbon cup offered great resistance to deterioration. For
instance, when an oxide was first fused in an oxygen blast, and then
mounted in the bulb, it melted very readily into a drop.
Generally, during the process of fusion, magnificent light effects were
noted, of which it would be difficult to give an adequate idea. Fig. 152
is intended to illustrate the effect observed with a ruby drop. At first
one may see a narrow funnel of white light projected against the top of
the globe, where it produces an irregularly outlined phosphorescent
patch. When the point of the ruby fuses, the phosphorescence becomes
very powerful; but as the atoms are projected with much greater speed
from the surface of the drop, soon the glass gets hot and "tired," and
now only the outer edge of the patch glows. In this manner an intensely
phosphorescent, sharply defined line, _l_, corresponding to the outline
of the drop, is produced, which spreads slowly over the globe as the
drop gets larger. When the mass begins to boil, small bubbles and
cavities are formed, which cause dark colored spots to sweep across the
globe. The bulb may be turned downward without fear of the drop falling
off, as the mass possesses considerable viscosity.
I may mention here another feature of some interest, which I believe to
have noted in the course of these experiments, though the observations
do not amount to a certitude. It _appeared_ that under the molecular
impact caused by the rapidly alternating potential, the body was fused
and maintained in that state at a lower temperature in a highly
exhausted bulb than was the case at normal pressure and application of
heat in the ordinary way--that is, at least, judging from the quantity
of the light emitted. One of the experiments performed may be mentioned
here by way of illustration. A small piece of pumice stone was stuck on
a platinum wire, and first melted to it in a gas burner. The wire was
next placed between two pieces of charcoal, and a burner applied, so as
to produce an intense heat, sufficient to melt down the pumice stone
into a small glass-like button. The platinum wire had to be taken of
sufficient thickness, to prevent its melting in the fire. While in the
charcoal fire, or when held in a burner to get a better idea of the
degree of heat, the button glowed with great brilliancy. The wire with
the button was then mounted in a bulb, and upon exhausting the same to a
high degree, the current was turned on slowly, so as to prevent the
cracking of the button. The button was heated to the point of fusion,
and when it melted, it did not, apparently, glow with the same
brilliancy as before, and this would indicate a lower temperature.
Leaving out of consideration the observer's possible, and even probable,
error, the question is, can a body under these conditions be brought
from a solid to a liquid state with the evolution of _less_ light?
When the potential of a body is rapidly alternated, it is certain that
the structure is jarred. When the potential is very high, although the
vibrations may be few--say 20,000 per second--the effect upon the
structure may be considerable. Suppose, for example, that a ruby is
melted into a drop by a steady application of energy. When it forms a
drop, it will emit visible and invisible waves, which will be in a
definite ratio, and to the eye the drop will appear to be of a certain
brilliancy. Next, suppose we diminish to any degree we choose the energy
steadily supplied, and, instead, supply energy which rises and falls
according to a certain law. Now, when the drop is formed, there will be
emitted from it three different kinds of vibrations--the ordinary
visible, and two kinds of invisible waves: that is, the ordinary dark
waves of all lengths, and, in addition, waves of a well defined
character. The latter would not exist by a steady supply of the energy;
still they help to jar and loosen the structure. If this really be the
case, then the ruby drop will emit relatively less visible and more
invisible waves than before. Thus it would seem that when a platinum
wire, for instance, is fused by currents alternating with extreme
rapidity, it emits at the point of fusion less light and more visible
radiation than it does when melted by a steady current, though the total
energy used up in the process of fusion is the same in both cases. Or,
to cite another example, a lamp filament is not capable of withstanding
as long with currents of extreme frequency as it does with steady
currents, assuming that it be worked at the same luminous intensity.
This means that for rapidly alternating currents the filament should be
shorter and thicker. The higher the frequency--that is, the greater the
departure from the steady flow--the worse it would be for the filament.
But if the truth of this remark were demonstrated, it would be erroneous
to conclude that such a refractory button as used in these bulbs would
be deteriorated quicker by currents of extremely high frequency than by
steady or low frequency currents. From experience I may say that just
the opposite holds good: the button withstands the bombardment better
with currents of very high frequency. But this is due to the fact that a
high frequency discharge passes through a rarefied gas with much greater
freedom than a steady or low frequency discharge, and this will mean
that with the former we can work with a lower potential or with a less
violent impact. As long, then, as the gas is of no consequence, a steady
or low frequency current is better; but as soon as the action of the gas
is desired and important, high frequencies are preferable.
In the course of these experiments a great many trials were made with
all kinds of carbon buttons. Electrodes made of ordinary carbon buttons
were decidedly more durable when the buttons were obtained by the
application of enormous pressure. Electrodes prepared by depositing
carbon in well known ways did not show up well; they blackened the globe
very quickly. From many experiences I conclude that lamp filaments
obtained in this manner can be advantageously used only with low
potentials and low frequency currents. Some kinds of carbon withstand so
well that, in order to bring them to the point of fusion, it is
necessary to employ very small buttons. In this case the observation is
rendered very difficult on account of the intense heat produced.
Nevertheless there can be no doubt that all kinds of carbon are fused
under the molecular bombardment, but the liquid state must be one of
great instability. Of all the bodies tried there were two which
withstood best--diamond and carborundum. These two showed up about
equally, but the latter was preferable for many reasons. As it is more
than likely that this body is not yet generally known, I will venture to
call your attention to it.
It has been recently produced by Mr. E. G. Acheson, of Monongahela City,
Pa., U. S. A. It is intended to replace ordinary diamond powder for
polishing precious stones, etc., and I have been informed that it
accomplishes this object quite successfully. I do not know why the name
"carborundum" has been given to it, unless there is something in the
process of its manufacture which justifies this selection. Through the
kindness of the inventor, I obtained a short while ago some samples
which I desired to test in regard to their qualities of phosphorescence
and capability of withstanding high degrees of heat.
Carborundum can be obtained in two forms--in the form of "crystals" and
of powder. The former appear to the naked eye dark colored, but are very
brilliant; the latter is of nearly the same color as ordinary diamond
powder, but very much finer. When viewed under a microscope the samples
of crystals given to me did not appear to have any definite form, but
rather resembled pieces of broken up egg coal of fine quality. The
majority were opaque, but there were some which were transparent and
colored. The crystals are a kind of carbon containing some impurities;
they are extremely hard, and withstand for a long time even an oxygen
blast. When the blast is directed against them they at first form a
cake of some compactness, probably in consequence of the fusion of
impurities they contain. The mass withstands for a very long time the
blast without further fusion; but a slow carrying off, or burning,
occurs, and, finally, a small quantity of a glass-like residue is left,
which, I suppose, is melted alumina. When compressed strongly they
conduct very well, but not as well as ordinary carbon. The powder, which
is obtained from the crystals in some way, is practically
non-conducting. It affords a magnificent polishing material for stones.
The time has been too short to make a satisfactory study of the
properties of this product, but enough experience has been gained in a
few weeks I have experimented upon it to say that it does possess some
remarkable properties in many respects. It withstands excessively high
degrees of heat, it is little deteriorated by molecular bombardment, and
it does not blacken the globe as ordinary carbon does. The only
difficulty which I have experienced in its use in connection with these
experiments was to find some binding material which would resist the
heat and the effect of the bombardment as successfully as carborundum
itself does.
I have here a number of bulbs which I have provided with buttons of
carborundum. To make such a button of carborundum crystals I proceed in
the following manner: I take an ordinary lamp filament and dip its point
in tar, or some other thick substance or paint which may be readily
carbonized. I next pass the point of the filament through the crystals,
and then hold it vertically over a hot plate. The tar softens and forms
a drop on the point of the filament, the crystals adhering to the
surface of the drop. By regulating the distance from the plate the tar
is slowly dried out and the button becomes solid. I then once more dip
the button in tar and hold it again over a plate until the tar is
evaporated, leaving only a hard mass which firmly binds the crystals.
When a larger button is required I repeat the process several times, and
I generally also cover the filament a certain distance below the button
with crystals. The button being mounted in a bulb, when a good vacuum
has been reached, first a weak and then a strong discharge is passed
through the bulb to carbonize the tar and expel all gases, and later it
is brought to a very intense incandescence.
When the powder is used I have found it best to proceed as follows: I
make a thick paint of carborundum and tar, and pass a lamp filament
through the paint. Taking then most of the paint off by rubbing the
filament against a piece of chamois leather, I hold it over a hot plate
until the tar evaporates and the coating becomes firm. I repeat this
process as many times as it is necessary to obtain a certain thickness
of coating. On the point of the coated filament I form a button in the
same manner.
There is no doubt that such a button--properly prepared under great
pressure--of carborundum, especially of powder of the best quality, will
withstand the effect of the bombardment fully as well as anything we
know. The difficulty is that the binding material gives way, and the
carborundum is slowly thrown off after some time. As it does not seem to
blacken the globe in the least, it might be found useful for coating the
filaments of ordinary incandescent lamps, and I think that it is even
possible to produce thin threads or sticks of carborundum which will
replace the ordinary filaments in an incandescent lamp. A carborundum
coating seems to be more durable than other coatings, not only because
the carborundum can withstand high degrees of heat, but also because it
seems to unite with the carbon better than any other material I have
tried. A coating of zirconia or any other oxide, for instance, is far
more quickly destroyed. I prepared buttons of diamond dust in the same
manner as of carborundum, and these came in durability nearest to those
prepared of carborundum, but the binding paste gave way much more
quickly in the diamond buttons; this, however, I attributed to the size
and irregularity of the grains of the diamond.
It was of interest to find whether carborundum possesses the quality of
phosphorescence. One is, of course, prepared to encounter two
difficulties: first, as regards the rough product, the "crystals," they
are good conducting, and it is a fact that conductors do not
phosphoresce; second, the powder, being exceedingly fine, would not be
apt to exhibit very prominently this quality, since we know that when
crystals, even such as diamond or ruby, are finely powdered, they lose
the property of phosphorescence to a considerable degree.
The question presents itself here, can a conductor phosphoresce? What is
there in such a body as a metal, for instance, that would deprive it of
the quality of phosphoresence, unless it is that property which
characterizes it as a conductor? For it is a fact that most of the
phosphorescent bodies lose that quality when they are sufficiently
heated to become more or less conducting. Then, if a metal be in a
large measure, or perhaps entirely, deprived of that property, it should
be capable of phosphoresence. Therefore it is quite possible that at
some extremely high frequency, when behaving practically as a
non-conductor, a metal or any other conductor might exhibit the quality
of phosphoresence, even though it be entirely incapable of
phosphorescing under the impact of a low-frequency discharge. There is,
however, another possible way how a conductor might at least _appear_ to
phosphoresce.
Considerable doubt still exists as to what really is phosphorescence,
and as to whether the various phenomena comprised under this head are
due to the same causes. Suppose that in an exhausted bulb, under the
molecular impact, the surface of a piece of metal or other conductor is
rendered strongly luminous, but at the same time it is found that it
remains comparatively cool, would not this luminosity be called
phosphorescence? Now such a result, theoretically at least, is possible,
for it is a mere question of potential or speed. Assume the potential of
the electrode, and consequently the speed of the projected atoms, to be
sufficiently high, the surface of the metal piece, against which the
atoms are projected, would be rendered highly incandescent, since the
process of heat generation would be incomparably faster than that of
radiating or conducting away from the surface of the collision. In the
eye of the observer a single impact of the atoms would cause an
instantaneous flash, but if the impacts were repeated with sufficient
rapidity, they would produce a continuous impression upon his retina. To
him then the surface of the metal would appear continuously incandescent
and of constant luminous intensity, while in reality the light would be
either intermittent, or at least changing periodically in intensity. The
metal piece would rise in temperature until equilibrium was
attained--that is, until the energy continuously radiated would equal
that intermittently supplied. But the supplied energy might under such
conditions not be sufficient to bring the body to any more than a very
moderate mean temperature, especially if the frequency of the atomic
impacts be very low--just enough that the fluctuation of the intensity
of the light emitted could not be detected by the eye. The body would
now, owing to the manner in which the energy is supplied, emit a strong
light, and yet be at a comparatively very low mean temperature. How
should the observer name the luminosity thus produced? Even if the
analysis of the light would teach him something definite, still he would
probably rank it under the phenomena of phosphorescence. It is
conceivable that in such a way both conducting and non-conducting bodies
may be maintained at a certain luminous intensity, but the energy
required would very greatly vary with the nature and properties of the
bodies.
These and some foregoing remarks of a speculative nature were made
merely to bring out curious features of alternate currents or electric
impulses. By their help we may cause a body to emit _more_ light, while
at a certain mean temperature, than it would emit if brought to that
temperature by a steady supply; and, again, we may bring a body to the
point of fusion, and cause it to emit _less_ light than when fused by
the application of energy in ordinary ways. It all depends on how we
supply the energy, and what kind of vibrations we set up; in one case
the vibrations are more, in the other less, adapted to affect our sense
of vision.
Some effects, which I had not observed before, obtained with carborundum
in the first trials, I attributed to phosphorescence, but in subsequent
experiments it appeared that it was devoid of that quality. The crystals
possess a noteworthy feature. In a bulb provided with a single electrode
in the shape of a small circular metal disc, for instance, at a certain
degree of exhaustion the electrode is covered with a milky film, which
is separated by a dark space from the glow filling the bulb. When the
metal disc is covered with carborundum crystals, the film is far more
intense, and snow-white. This I found later to be merely an effect of
the bright surface of the crystals, for when an aluminum electrode was
highly polished, it exhibited more or less the same phenomenon. I made a
number of experiments with the samples of crystals obtained, principally
because it would have been of special interest to find that they are
capable of phosphorescence, on account of their being conducting. I
could not produce phosphorescence distinctly, but I must remark that a
decisive opinion cannot be formed until other experimenters have gone
over the same ground.
The powder behaved in some experiments as though it contained alumina,
but it did not exhibit with sufficient distinctness the red of the
latter. Its dead color brightens considerably under the molecular
impact, but I am now convinced it does not phosphoresce. Still, the
tests with the powder are not conclusive, because powdered carborundum
probably does not behave like a phosphorescent sulphide, for example,
which could be finely powdered without impairing the phosphorescence,
but rather like powdered ruby or diamond, and therefore it would be
necessary, in order to make a decisive test, to obtain it in a large
lump and polish up the surface.
If the carborundum proves useful in connection with these and similar
experiments, its chief value will be found in the production of
coatings, thin conductors, buttons, or other electrodes capable of
withstanding extremely high degrees of heat.
The production of a small electrode, capable of withstanding enormous
temperatures, I regard as of the greatest importance in the manufacture
of light. It would enable us to obtain, by means of currents of very
high frequencies, certainly 20 times, if not more, the quantity of light
which is obtained in the present incandescent lamp by the same
expenditure of energy. This estimate may appear to many exaggerated, but
in reality I think it is far from being so. As this statement might be
misunderstood, I think it is necessary to expose clearly the problem
with which, in this line of work, we are confronted, and the manner in
which, in my opinion, a solution will be arrived at.
Any one who begins a study of the problem will be apt to think that what
is wanted in a lamp with an electrode is a very high degree of
incandescence of the electrode. There he will be mistaken. The high
incandescence of the button is a necessary evil, but what is really
wanted is the high incandescence of the gas surrounding the button. In
other words, the problem in such a lamp is to bring a mass of gas to the
highest possible incandescence. The higher the incandescence, the
quicker the mean vibration, the greater is the economy of the light
production. But to maintain a mass of gas at a high degree of
incandescence in a glass vessel, it will always be necessary to keep the
incandescent mass away from the glass; that is, to confine it as much as
possible to the central portion of the globe.
In one of the experiments this evening a brush was produced at the end
of a wire. The brush was a flame, a source of heat and light. It did not
emit much perceptible heat, nor did it glow with an intense light; but
is it the less a flame because it does not scorch my hand? Is it the
less a flame because it does not hurt my eyes by its brilliancy? The
problem is precisely to produce in the bulb such a flame, much smaller
in size, but incomparably more powerful. Were there means at hand for
producing electric impulses of a sufficiently high frequency, and for
transmitting them, the bulb could be done away with, unless it were used
to protect the electrode, or to economize the energy by confining the
heat. But as such means are not at disposal, it becomes necessary to
place the terminal in the bulb and rarefy the air in the same. This is
done merely to enable the apparatus to perform the work which it is not
capable of performing at ordinary air pressure. In the bulb we are able
to intensify the action to any degree--so far that the brush emits a
powerful light.
The intensity of the light emitted depends principally on the frequency
and potential of the impulses, and on the electric density on the
surface of the electrode. It is of the greatest importance to employ the
smallest possible button, in order to push the density very far. Under
the violent impact of the molecules of the gas surrounding it, the small
electrode is of course brought to an extremely high temperature, but
around it is a mass of highly incandescent gas, a flame photosphere,
many hundred times the volume of the electrode. With a diamond,
carborundum or zirconia button the photosphere can be as much as one
thousand times the volume of the button. Without much reflection one
would think that in pushing so far the incandescence of the electrode it
would be instantly volatilized. But after a careful consideration one
would find that, theoretically, it should not occur, and in this
fact--which, moreover, is experimentally demonstrated--lies principally
the future value of such a lamp.
At first, when the bombardment begins, most of the work is performed on
the surface of the button, but when a highly conducting photosphere is
formed the button is comparatively relieved. The higher the
incandescence of the photosphere, the more it approaches in conductivity
to that of the electrode, and the more, therefore, the solid and the gas
form one conducting body. The consequence is that the further the
incandescence is forced the more work, comparatively, is performed on
the gas, and the less on the electrode. The formation of a powerful
photosphere is consequently the very means for protecting the electrode.
This protection, of course, is a relative one, and it should not be
thought that by pushing the incandescence higher the electrode is
actually less deteriorated. Still, theoretically, with extreme
frequencies, this result must be reached, but probably at a temperature
too high for most of the refractory bodies known. Given, then, an
electrode which can withstand to a very high limit the effect of the
bombardment and outward strain, it would be safe, no matter how much it
was forced beyond that limit. In an incandescent lamp quite different
considerations apply. There the gas is not at all concerned; the whole
of the work is performed on the filament; and the life of the lamp
diminishes so rapidly with the increase of the degree of incandescence
that economical reasons compel us to work it at a low incandescence. But
if an incandescent lamp is operated with currents of very high
frequency, the action of the gas cannot be neglected, and the rules for
the most economical working must be considerably modified.
In order to bring such a lamp with one or two electrodes to a great
perfection, it is necessary to employ impulses of very high frequency.
The high frequency secures, among others, two chief advantages, which
have a most important bearing upon the economy of the light production.
First, the deterioration of the electrode is reduced by reason of the
fact that we employ a great many small impacts, instead of a few violent
ones, which quickly shatter the structure; secondly, the formation of a
large photosphere is facilitated.
In order to reduce the deterioration of the electrode to the minimum, it
is desirable that the vibration be harmonic, for any suddenness hastens
the process of destruction. An electrode lasts much longer when kept at
incandescence by currents, or impulses, obtained from a high frequency
alternator, which rise and fall more or less harmonically, than by
impulses obtained from a disruptive discharge coil. In the latter case
there is no doubt that most of the damage is done by the fundamental
sudden discharges.
One of the elements of loss in such a lamp is the bombardment of the
globe. As the potential is very high, the molecules are projected with
great speed; they strike the glass, and usually excite a strong
phosphorescence. The effect produced is very pretty, but for economical
reasons it would be perhaps preferable to prevent, or at least reduce to
a minimum, the bombardment against the globe, as in such case it is, as
a rule, not the object to excite phosphorescence, and as some loss of
energy results from the bombardment. This loss in the bulb is
principally dependent on the potential of the impulses and on the
electric density on the surface of the electrode. In employing very high
frequencies the loss of energy by the bombardment is greatly reduced,
for, first, the potential needed to perform a given amount of work is
much smaller; and, secondly, by producing a highly conducting
photosphere around the electrode, the same result is obtained as though
the electrode were much larger, which is equivalent to a smaller
electric density. But be it by the diminution of the maximum potential
or of the density, the gain is effected in the same manner, namely, by
avoiding violent shocks, which strain the glass much beyond its limit of
elasticity. If the frequency could be brought high enough, the loss due
to the imperfect elasticity of the glass would be entirely negligible.
The loss due to bombardment of the globe may, however, be reduced by
using two electrodes instead of one. In such case each of the electrodes
may be connected to one of the terminals; or else, if it is preferable
to use only one wire, one electrode may be connected to one terminal and
the other to the ground or to an insulated body of some surface, as, for
instance, a shade on the lamp. In the latter case, unless some judgment
is used, one of the electrodes might glow more intensely than the other.
But on the whole I find it preferable, when using such high frequencies,
to employ only one electrode and one connecting wire. I am convinced
that the illuminating device of the near future will not require for its
operation more than one lead, and, at any rate, it will have no
leading-in wire, since the energy required can be as well transmitted
through the glass. In experimental bulbs the leading-in wire is not
generally used on account of convenience, as in employing condenser
coatings in the manner indicated in Fig. 151, for example, there is some
difficulty in fitting the parts, but these difficulties would not exist
if a great many bulbs were manufactured; otherwise the energy can be
conveyed through the glass as well as through a wire, and with these
high frequencies the losses are very small. Such illustrating devices
will necessarily involve the use of very high potentials, and this, in
the eyes of practical men, might be an objectionable feature. Yet, in
reality, high potentials are not objectionable--certainly not in the
least so far as the safety of the devices is concerned.
There are two ways of rendering an electric appliance safe. One is to
use low potentials, the other is to determine the dimensions of the
apparatus so that it is safe, no matter how high a potential is used. Of
the two, the latter seems to me the better way, for then the safety is
absolute, unaffected by any possible combination of circumstances which
might render even a low-potential appliance dangerous to life and
property. But the practical conditions require not only the judicious
determination of the dimensions of the apparatus; they likewise
necessitate the employment of energy of the proper kind. It is easy, for
instance, to construct a transformer capable of giving, when operated
from an ordinary alternate current machine of low tension, say 50,000
volts, which might be required to light a highly exhausted
phosphorescent tube, so that, in spite of the high potential, it is
perfectly safe, the shock from it producing no inconvenience. Still such
a transformer would be expensive, and in itself inefficient; and,
besides, what energy was obtained from it would not be economically used
for the production of light. The economy demands the employment of
energy in the form of extremely rapid vibrations. The problem of
producing light has been likened to that of maintaining a certain
high-pitch note by means of a bell. It should be said a _barely audible_
note; and even these words would not express it, so wonderful is the
sensitiveness of the eye. We may deliver powerful blows at long
intervals, waste a good deal of energy, and still not get what we want;
or we may keep up the note by delivering frequent taps, and get nearer
to the object sought by the expenditure of much less energy. In the
production of light, as far as the illuminating device is concerned,
there can be only one rule--that is, to use as high frequencies as can
be obtained; but the means for the production and conveyance of impulses
of such character impose, at present at least, great limitations. Once
it is decided to use very high frequencies, the return wire becomes
unnecessary, and all the appliances are simplified. By the use of
obvious means the same result is obtained as though the return wire were
used. It is sufficient for this purpose to bring in contact with the
bulb, or merely in the vicinity of the same, an insulated body of some
surface. The surface need, of course, be the smaller, the higher the
frequency and potential used, and necessarily, also, the higher the
economy of the lamp or other device.
This plan of working has been resorted to on several occasions this
evening. So, for instance, when the incandescence of a button was
produced by grasping the bulb with the hand, the body of the
experimenter merely served to intensify the action. The bulb used was
similar to that illustrated in Fig. 148, and the coil was excited to a
small potential, not sufficient to bring the button to incandescence
when the bulb was hanging from the wire; and incidentally, in order to
perform the experiment in a more suitable manner, the button was taken
so large that a perceptible time had to elapse before, upon grasping the
bulb, it could be rendered incandescent. The contact with the bulb was,
of course, quite unnecessary. It is easy, by using a rather large bulb
with an exceedingly small electrode, to adjust the conditions so that
the latter is brought to bright incandescence by the mere approach of
the experimenter within a few feet of the bulb, and that the
incandescence subsides upon his receding.
[Illustration: FIG. 153.]
[Illustration: FIG. 154.]
In another experiment, when phosphorescence was excited, a similar bulb
was used. Here again, originally, the potential was not sufficient to
excite phosphorescence until the action was intensified--in this case,
however, to present a different feature, by touching the socket with a
metallic object held in the hand. The electrode in the bulb was a carbon
button so large that it could not be brought to incandescence, and
thereby spoil the effect produced by phosphorescence.
Again, in another of the early experiments, a bulb was used, as
illustrated in Fig. 141. In this instance, by touching the bulb with one
or two fingers, one or two shadows of the stem inside were projected
against the glass, the touch of the finger producing the same results as
the application of an external negative electrode under ordinary
circumstances.
In all these experiments the action was intensified by augmenting the
capacity at the end of the lead connected to the terminal. As a rule, it
is not necessary to resort to such means, and would be quite unnecessary
with still higher frequencies; but when it _is_ desired, the bulb, or
tube, can be easily adapted to the purpose.
In Fig. 153, for example, an experimental bulb, L, is shown, which
is provided with a neck, _n_, on the top, for the application of an
external tinfoil coating, which may be connected to a body of larger
surface. Such a lamp as illustrated in Fig. 154 may also be lighted by
connecting the tinfoil coating on the neck _n_ to the terminal, and the
leading-in wire, _w_, to an insulated plate. If the bulb stands in a
socket upright, as shown in the cut, a shade of conducting material may
be slipped in the neck, _n_, and the action thus magnified.
A more perfected arrangement used in some of these bulbs is illustrated
in Fig. 155. In this case the construction of the bulb is as shown and
described before, when reference was made to Fig. 148. A zinc sheet, Z,
with a tubular extension, T, is applied over the metallic socket, S.
The bulb hangs downward from the terminal, _t_, the zinc sheet, Z,
performing the double office of intensifier and reflector. The reflector
is separated from the terminal, _t_, by an extension of the insulating
plug, P.
A similar disposition with a phosphorescent tube is illustrated in
Fig. 156. The tube, T, is prepared from two short tubes of different
diameter, which are sealed on the ends. On the lower end is placed an
inside conducting coating, C, which connects to the wire _w_. The wire
has a hook on the upper end for suspension, and passes through the
centre of the inside tube, which is filled with some good and tightly
packed insulator. On the outside of the upper end of the tube, T, is
another conducting coating, C_{1}, upon which is slipped a metallic
reflector Z, which should be separated by a thick insulation from the
end of wire _w_.
The economical use of such a reflector or intensifier would require that
all energy supplied to an air condenser should be recoverable, or, in
other words, that there should not be any losses, neither in the
gaseous medium nor through its action elsewhere. This is far from being
so, but, fortunately, the losses may be reduced to anything desired. A
few remarks are necessary on this subject, in order to make the
experiences gathered in the course of these investigations perfectly
clear.
[Illustration: FIG. 155.]
Suppose a small helix with many well insulated turns, as in experiment
Fig. 146, has one of its ends connected to one of the terminals of the
induction coil, and the other to a metal plate, or, for the sake of
simplicity, a sphere, insulated in space. When the coil is set to work,
the potential of the sphere is alternated, and a small helix now behaves
as though its free end were connected to the other terminal of the
induction coil. If an iron rod be held within a small helix, it is
quickly brought to a high temperature, indicating the passage of a
strong current through the helix. How does the insulated sphere act in
this case? It can be a condenser, storing and returning the energy
supplied to it, or it can be a mere sink of energy, and the conditions
of the experiment determine whether it is rather one than the other. The
sphere being charged to a high potential, it acts inductively upon the
surrounding air, or whatever gaseous medium there might be. The
molecules, or atoms, which are near the sphere, are of course more
attracted, and move through a greater distance than the farther ones.
When the nearest molecules strike the sphere, they are repelled, and
collisions occur at all distances within the inductive action of the
sphere. It is now clear that, if the potential be steady, but little
loss of energy can be caused in this way, for the molecules which are
nearest to the sphere, having had an additional charge imparted to them
by contact, are not attracted until they have parted, if not with all,
at least with most of the additional charge, which can be accomplished
only after a great many collisions. From the fact, that with a steady
potential there is but little loss in dry air, one must come to such a
conclusion. When the potential of a sphere, instead of being steady, is
alternating, the conditions are entirely different. In this case a
rhythmical bombardment occurs, no matter whether the molecules, after
coming in contact with the sphere, lose the imparted charge or not; what
is more, if the charge is not lost, the impacts are only the more
violent. Still, if the frequency of the impulses be very small, the loss
caused by the impacts and collisions would not be serious, unless the
potential were excessive. But when extremely high frequencies and more
or less high potentials are used, the loss may very great. The total
energy lost per unit of time is proportionate to the product of the
number of impacts per second, or the frequency and the energy lost in
each impact. But the energy of an impact must be proportionate to the
square of the electric density of the sphere, since the charge imparted
to the molecule is proportionate to that density. I conclude from this
that the total energy lost must be proportionate to the product of the
frequency and the square of the electric density; but this law needs
experimental confirmation. Assuming the preceding considerations to be
true, then, by rapidly alternating the potential of a body immersed in
an insulating gaseous medium, any amount of energy may be dissipated
into space. Most of that energy then, I believe, is not dissipated in
the form of long ether waves, propagated to considerable distance, as is
thought most generally, but is consumed--in the case of an insulated
sphere, for example--in impact and collisional losses--that is, heat
vibrations--on the surface and in the vicinity of the sphere. To reduce
the dissipation, it is necessary to work with a small electric
density--the smaller, the higher the frequency.
[Illustration: FIG. 156.]
But since, on the assumption before made, the loss is diminished with
the square of the density, and since currents of very high frequencies
involve considerable waste when transmitted through conductors, it
follows that, on the whole, it is better to employ one wire than two.
Therefore, if motors, lamps, or devices of any kind are perfected,
capable of being advantageously operated by currents of extremely high
frequency, economical reasons will make it advisable to use only one
wire, especially if the distances are great.
When energy is absorbed in a condenser, the same behaves as though its
capacity were increased. Absorption always exists more or less, but
generally it is small and of no consequence as long as the frequencies
are not very great. In using extremely high frequencies, and,
necessarily in such case, also high potentials, the absorption--or, what
is here meant more particularly by this term, the loss of energy due to
the presence of a gaseous medium--is an important factor to be
considered, as the energy absorbed in the air condenser may be any
fraction of the supplied energy. This would seem to make it very
difficult to tell from the measured or computed capacity of an air
condenser its actual capacity or vibration period, especially if the
condenser is of very small surface and is charged to a very high
potential. As many important results are dependent upon the correctness
of the estimation of the vibration period, this subject demands the most
careful scrutiny of other investigators. To reduce the probable error as
much as possible in experiments of the kind alluded to, it is advisable
to use spheres or plates of large surface, so as to make the density
exceedingly small. Otherwise, when it is practicable, an oil condenser
should be used in preference. In oil or other liquid dielectrics there
are seemingly no such losses as in gaseous media. It being impossible to
exclude entirely the gas in condensers with solid dielectrics, such
condensers should be immersed in oil, for economical reasons, if nothing
else; they can then be strained to the utmost, and will remain cool. In
Leyden jars the loss due to air is comparatively small, as the tinfoil
coatings are large, close together, and the charged surfaces not
directly exposed; but when the potentials are very high, the loss may be
more or less considerable at, or near, the upper edge of the foil, where
the air is principally acted upon. If the jar be immersed in boiled-out
oil, it will be capable of performing four times the amount of work
which it can for any length of time when used in the ordinary way, and
the loss will be inappreciable.
It should not be thought that the loss in heat in an air condenser is
necessarily associated with the formation of _visible_ streams or
brushes. If a small electrode, inclosed in an unexhausted bulb, is
connected to one of the terminals of the coil, streams can be seen to
issue from the electrode, and the air in the bulb is heated; if instead
of a small electrode a large sphere is inclosed in the bulb, no streams
are observed, still the air is heated.
Nor should it be thought that the temperature of an air condenser would
give even an approximate idea of the loss in heat incurred, as in such
case heat must be given off much more quickly, since there is, in
addition to the ordinary radiation, a very active carrying away of heat
by independent carriers going on, and since not only the apparatus, but
the air at some distance from it is heated in consequence of the
collisions which must occur.
Owing to this, in experiments with such a coil, a rise of temperature
can be distinctly observed only when the body connected to the coil is
very small. But with apparatus on a larger scale, even a body of
considerable bulk would be heated, as, for instance, the body of a
person; and I think that skilled physicians might make observations of
utility in such experiments, which, if the apparatus were judiciously
designed, would not present the slightest danger.
A question of some interest, principally to meteorologists, presents
itself here. How does the earth behave? The earth is an air condenser,
but is it a perfect or a very imperfect one--a mere sink of energy?
There can be little doubt that to such small disturbance as might be
caused in an experiment, the earth behaves as an almost perfect
condenser. But it might be different when its charge is set in vibration
by some sudden disturbance occurring in the heavens. In such case, as
before stated, probably only little of the energy of the vibrations set
up would be lost into space in the form of long ether radiations, but
most of the energy, I think, would spend itself in molecular impacts and
collisions, and pass off into space in the form of short heat, and
possibly light, waves. As both the frequency of the vibrations of the
charge and the potential are in all probability excessive, the energy
converted into heat may be considerable. Since the density must be
unevenly distributed, either in consequence of the irregularity of the
earth's surface, or on account of the condition of the atmosphere in
various places, the effect produced would accordingly vary from place to
place. Considerable variations in the temperature and pressure of the
atmosphere may in this manner be caused at any point of the surface of
the earth. The variations may be gradual or very sudden, according to
the nature of the general disturbance, and may produce rain and storms,
or locally modify the weather in any way.
From the remarks before made, one may see what an important factor of
loss the air in the neighborhood of a charged surface becomes when the
electric density is great and the frequency of the impulses excessive.
But the action, as explained, implies that the air is insulating--that
is, that it is composed of independent carriers immersed in an
insulating medium. This is the case only when the air is at something
like ordinary or greater, or at extremely small, pressure. When the air
is slightly rarefied and conducting, then true conduction losses occur
also. In such case, of course, considerable energy may be dissipated
into space even with a steady potential, or with impulses of low
frequency, if the density is very great.
When the gas is at very low pressure, an electrode is heated more
because higher speeds can be reached. If the gas around the electrode is
strongly compressed, the displacements, and consequently the speeds, are
very small, and the heating is insignificant. But if in such case the
frequency could be sufficiently increased, the electrode would be
brought to a high temperature as well as if the gas were at very low
pressure; in fact, exhausting the bulb is only necessary because we
cannot produce, (and possibly not convey) currents of the required
frequency.
Returning to the subject of electrode lamps, it is obviously of
advantage in such a lamp to confine as much as possible the heat to the
electrode by preventing the circulation of the gas in the bulb. If a
very small bulb be taken, it would confine the heat better than a large
one, but it might not be of sufficient capacity to be operated from the
coil, or, if so, the glass might get too hot. A simple way to improve in
this direction is to employ a globe of the required size, but to place a
small bulb, the diameter of which is properly estimated, over the
refractory button contained in the globe. This arrangement is
illustrated in Fig. 157.
[Illustration: FIG. 157.]
[Illustration: FIG. 158.]
The globe L has in this case a large neck _n_, allowing the small bulb
_b_ to slip through. Otherwise the construction is the same as shown in
Fig. 147, for example. The small bulb is conveniently supported upon the
stem _s_, carrying the refractory button _m_. It is separated from the
aluminum tube _a_ by several layers of mica M, in order to prevent the
cracking of the neck by the rapid heating of the aluminum tube upon a
sudden turning on of the current. The inside bulb should be as small as
possible when it is desired to obtain light only by incandescence of the
electrode. If it is desired to produce phosphorescence, the bulb should
be larger, else it would be apt to get too hot, and the phosphorescence
would cease. In this arrangement usually only the small bulb shows
phosphorescence, as there is practically no bombardment against the
outer globe. In some of these bulbs constructed as illustrated in Fig.
157, the small tube was coated with phosphorescent paint, and beautiful
effects were obtained. Instead of making the inside bulb large, in order
to avoid undue heating, it answers the purpose to make the electrode _m_
larger. In this case the bombardment is weakened by reason of the
smaller electric density.
Many bulbs were constructed on the plan illustrated in Fig. 158. Here a
small bulb _b_, containing the refractory button _m_, upon being
exhausted to a very high degree was sealed in a large globe L, which was
then moderately exhausted and sealed off. The principal advantage of
this construction was that it allowed of reaching extremely high vacua,
and, at the same time of using a large bulb. It was found, in the course
of experiments with bulbs such as illustrated in Fig. 158, that it was
well to make the stem _s_, near the seal at _e_, very thick, and the
leading-in wire _w_ thin, as it occurred sometimes that the stem at _e_
was heated and the bulb was cracked. Often the outer globe L was
exhausted only just enough to allow the discharge to pass through, and
the space between the bulbs appeared crimson, producing a curious
effect. In some cases, when the exhaustion in globe L was very low, and
the air good conducting, it was found necessary, in order to bring the
button _m_ to high incandescence, to place, preferably on the upper part
of the neck of the globe, a tinfoil coating which was connected to an
insulated body, to the ground, or to the other terminal of the coil, as
the highly conducting air weakened the effect somewhat, probably by
being acted upon inductively from the wire _w_, where it entered the
bulb at _e_. Another difficulty--which, however, is always present when
the refractory button is mounted in a very small bulb--existed in the
construction illustrated in Fig. 158, namely, the vacuum in the bulb _b_
would be impaired in a comparatively short time.
The chief idea in the two last described constructions was to confine
the heat to the central portion of the globe by preventing the exchange
of air. An advantage is secured, but owing to the heating of the inside
bulb and slow evaporation of the glass, the vacuum is hard to maintain,
even if the construction illustrated in Fig. 157 be chosen, in which
both bulbs communicate.
But by far the better way--the ideal way--would be to reach sufficiently
high frequencies. The higher the frequency, the slower would be the
exchange of the air, and I think that a frequency may be reached, at
which there would be no exchange whatever of the air molecules around
the terminal. We would then produce a flame in which there would be no
carrying away of material, and a queer flame it would be, for it would
be rigid! With such high frequencies the inertia of the particles would
come into play. As the brush, or flame, would gain rigidity in virtue of
the inertia of the particles, the exchange of the latter would be
prevented. This would necessarily occur, for, the number of impulses
being augmented, the potential energy of each would diminish, so that
finally only atomic vibrations could be set up, and the motion of
translation through measurable space would cease. Thus an ordinary gas
burner connected to a source of rapidly alternating potential might have
its efficiency augmented to a certain limit, and this for two
reasons--because of the additional vibration imparted, and because of a
slowing down of the process of carrying off. But the renewal being
rendered difficult, a renewal being necessary to maintain the _burner_,
a continued increase of the frequency of the impulses, assuming they
could be transmitted to and impressed upon the flame, would result in
the "extinction" of the latter, meaning by this term only the cessation
of the chemical process.
I think, however, that in the case of an electrode immersed in a fluid
insulating medium, and surrounded by independent carriers of electric
charges, which can be acted upon inductively, a sufficient high
frequency of the impulses would probably result in a gravitation of the
gas all around toward the electrode. For this it would be only necessary
to assume that the independent bodies are irregularly shaped; they would
then turn toward the electrode their side of the greatest electric
density, and this would be a position in which the fluid resistance to
approach would be smaller than that offered to the receding.