aluminium, 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. 12 and 13.
In Fig. 12 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 aluminium 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.
[Illustration: FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.]
The construction shown in Fig. 13 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 conducting 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. 14.--FORMS AND PHASES OF THE ROTATING BRUSH.]
Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
Fig. 14 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. 12 and 13) is exhausted to a very high
degree, generally the bulb is not excited upon connecting the wire w
(Fig. 12) or the tinfoil coating of the bulb (Fig. 13) 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. 15. From this stage the phenomenon will
gradually pass to that indicated in Fig. 16, 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. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING
BRUSH.]
When the brush assumes the form indicated in Fig. 16, it maybe 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
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 in
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 result. 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 aluminium 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 overlap partly 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. 17), comprising a coil and iron core, and a freely movable copper
disc in proximity to the latter.
[Illustration: FIG. 17.--SINGLE WIRE AND "NO-WIRE" MOTOR.]
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 subtile 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
alternate currents 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 inclosed in a small bulb, because of the better
confinement of heat in this case.