upon which I have glued two very thin wires ww, so as to form a name.
The wires may be bare or covered with the best insulation--it is
immaterial for the success of the experiment. Well insulated wires, if
anything, are preferable. On the back of each plate, indicated by the
shaded portion, is a tinfoil coating tt. The plates are placed in line
at a sufficient distance to prevent a spark passing from one to the
other wire. The two tinfoil coatings I have joined by a conductor C,
and the two wires I presently connect to the terminals of the coil. It
is now easy, by varying the strength and frequency of the currents
through the primary, to find a point at which, the capacity of the
system is best suited to the conditions, and the wires become so
strongly luminous that, when the light in the room is turned off the
name formed by them appears in brilliant letters.
[Illustration: FIG. 9.--WIRES RENDERED INTENSELY LUMINOUS.]
It is perhaps preferable to perform this experiment with a coil
operated from an alternator of high frequency, as then, owing to the
harmonic rise and fall, the streams are very uniform, though they are
less abundant then when produced with such a coil as the present. This
experiment, however, may be performed with low frequencies, but much
less satisfactorily.
[Illustration: FIG. 10.--LUMINOUS DISCS.]
When two wires, attached to the terminals of the coil, are set at the
proper distance, the streams between them may be so intense as to
produce a continuous luminous sheet. To show this phenomenon I have
here two circles, C and c (Fig. 10), of rather stout wire, one being
about 80 centimetres and the other 30 centimetres in diameter. To each
of the terminals of the coil I attach one of the circles. The
supporting wires are so bent that the circles may be placed in the
same plane, coinciding as nearly as possible. When the light in the
room is turned off and the coil set to work, you see the whole space
between the wires uniformly filled with streams, forming a luminous
disc, which could be seen from a considerable distance, such is the
intensity of the streams. The outer circle could have been much larger
than the present one; in fact, with this coil I have used much larger
circles, and I have been able to produce a strongly luminous sheet,
covering an area of more than one square metre, which is a remarkable
effect with this very small coil. To avoid uncertainty, the circle has
been taken smaller, and the area is now about 0.43 square metre.
The frequency of the vibration, and the quickness of succession of the
sparks between the knobs, affect to a marked degree the appearance of
the streams. When the frequency is very low, the air gives way in more
or less the same manner, as by a steady difference of potential, and
the streams consist of distinct threads, generally mingled with thin
sparks, which probably correspond to the successive discharges
occurring between the knobs. But when the frequency is extremely high,
and the arc of the discharge produces a very _loud_ but _smooth_
sound--showing both that oscillation takes place and that the sparks
succeed each other with great rapidity--then the luminous streams
formed are perfectly uniform. To reach this result very small coils
and jars of small capacity should be used. I take two tubes of thick
Bohemian glass, about 5 centimetres in diameter and 20 centimetres
long. In each of the tubes I slip a primary of very thick copper wire.
On the top of each tube I wind a secondary of much thinner
gutta-percha covered wire. The two secondaries I connect in series,
the primaries preferably in multiple arc. The tubes are then placed in
a large glass vessel, at a distance of 10 to 15 centimetres from each
other, on insulating supports, and the vessel is filled with boiled
out oil, the oil reaching about an inch above the tubes. The free ends
of the secondary are lifted out of the oil and placed parallel to each
other at a distance of about 10 centimetres. The ends which are
scraped should be dipped in the oil. Two four-pint jars joined in
series may be used to discharge through the primary. When the
necessary adjustments in the length and distance of the wires above
the oil and in the arc of discharge are made, a luminous sheet is
produced between the wires which is perfectly smooth and textureless,
like the ordinary discharge through a moderately exhausted tube.
I have purposely dwelt upon this apparently insignificant experiment.
In trials of this kind the experimenter arrives at the startling
conclusion that, to pass ordinary luminous discharges through gases,
no particular degree of exhaustion is needed, but that the gas may be
at ordinary or even greater pressure. To accomplish this, a very high
frequency is essential; a high potential is likewise required, but
this is a merely incidental necessity. These experiments teach us
that, in endeavoring to discover novel methods of producing light by
the agitation of atoms, or molecules, of a gas, we need not limit our
research to the vacuum tube, but may look forward quite seriously to
the possibility of obtaining the light effects without the use of any
vessel whatever, with air at ordinary pressure.
Such discharges of very high frequency, which render luminous the air
at ordinary pressures, we have probably often occasion to witness in
Nature. I have no doubt that if, as many believe, the aurora borealis
is produced by sudden cosmic disturbances, such as eruptions at the
sun's surface, which set the electrostatic charge of the earth in an
extremely rapid vibration, the red glow observed is not confined to
the upper rarefied strata of the air, but the discharge traverses, by
reason of its very high frequency, also the dense atmosphere in the
form of a _glow_, such as we ordinarily produce in a slightly
exhausted tube. If the frequency were very low, or even more so, if
the charge were not at all vibrating, the dense air would break down
as in a lightning discharge. Indications of such breaking down of the
lower dense strata of the air have been repeatedly observed at the
occurrence of this marvelous phenomenon; but if it does occur, it can
only be attributed to the fundamental disturbances, which are few in
number, for the vibration produced by them would be far too rapid to
allow a disruptive break. It is the original and irregular impulses
which affect the instruments; the superimposed vibrations probably
pass unnoticed.
When an ordinary low frequency discharge is passed through moderately
rarefied air, the air assumes a purplish hue. If by some means or
other we increase the intensity of the molecular, or atomic,
vibration, the gas changes to a white color. A similar change occurs
at ordinary pressures with electric impulses of very high frequency.
If the molecules of the air around a wire are moderately agitated, the
brush formed is reddish or violet; if the vibration is rendered
sufficiently intense, the streams become white. We may accomplish this
in various ways. In the experiment before shown with the two wires
across the room, I have endeavored to secure the result by pushing to
a high value both the frequency and potential: in the experiment with
the thin wires glued on the rubber plate I have concentrated the
action upon a very small surface--in other words, I have worked with a
great electric density.
A most curious form of discharge is observed with such a coil when the
frequency and potential are pushed to the extreme limit. To perform
the experiment, every part of the coil should be heavily insulated,
and only two small spheres--or, better still, two sharp-edged metal
discs (dd, Fig. 11) of no more than a few centimetres in
diameter--should be exposed to the air. The coil here used is immersed
in oil, and the ends of the secondary reaching out of the oil are
covered with an air-tight cover of hard rubber of great thickness.
All cracks, if there are any, should be carefully stopped up, so that
the brush discharge cannot form anywhere except on the small spheres
or plates which are exposed to the air. In this case, since there are
no large plates or other bodies of capacity attached to the terminals,
the coil is capable of an extremely rapid vibration. The potential may
be raised by increasing, as far as the experimenter judges proper, the
rate of change of the primary current. With a coil not widely
differing from the present, it is best to connect the two primaries in
multiple arc; but if the secondary should have a much greater number
of turns the primaries should preferably be used in series, as
otherwise the vibration might be too fast for the secondary. It occurs
under these conditions that misty white streams break forth from the
edges of the discs and spread out phantom-like into space. With this
coil, when fairly well produced, they are about 25 to 30 centimetres
long. When the hand is held against them no sensation is produced, and
a spark, causing a shock, jumps from the terminal only upon the hand
being brought much nearer. If the oscillation of the primary current
is rendered intermittent by some means or other, there is a
corresponding throbbing of the streams, and now the hand or other
conducting object may be brought in still greater proximity to the
terminal without a spark being caused to jump.
[Illustration: FIG. 11.--PHANTOM STREAMS.]
Among the many beautiful phenomena which may be produced with such a
coil I have here selected only those which appear to possess some
features of novelty, and lead us to some conclusions of interest. One
will not find it at all difficult to produce in the laboratory, by
means of it, many other phenomena which appeal to the eye even more
than these here shown, but present no particular feature of novelty.
Early experimenters describe the display of sparks produced by an
ordinary large induction coil upon an insulating plate separating the
terminals. Quite recently Siemens performed some experiments in which
fine effects were obtained, which were seen by many with interest. No
doubt large coils, even if operated with currents of low frequencies,
are capable of producing beautiful effects. But the largest coil ever
made could not, by far, equal the magnificent display of streams and
sparks obtained from such a disruptive discharge coil when properly
adjusted. To give an idea, a coil such as the present one will cover
easily a plate of 1 metre in diameter completely with the streams. The
best way to perform such experiments is to take a very thin rubber or
a glass plate and glue on one side of it a narrow ring of tinfoil of
very large diameter, and on the other a circular washer, the centre of
the latter coinciding with that of the ring, and the surfaces of both
being preferably equal, so as to keep the coil well balanced. The
washer and ring should be connected to the terminals by heavily
insulated thin wires. It is easy in observing the effect of the
capacity to produce a sheet of uniform streams, or a fine network of
thin silvery threads, or a mass of loud brilliant sparks, which
completely cover the plate.
Since I have advanced the idea of the conversion by means of the
disruptive discharge, in my paper before the American Institute of
Electrical Engineers at the beginning of the past year, the interest
excited in it has been considerable. It affords us a means for
producing any potentials by the aid of inexpensive coils operated from
ordinary systems of distribution, and--what is perhaps more
appreciated--it enables us to convert currents of any frequency into
currents of any other lower or higher frequency. But its chief value
will perhaps be found in the help which it will afford us in the
investigations of the phenomena of phosphorescence, which a disruptive
discharge coil is capable of exciting in innumerable cases where
ordinary coils, even the largest, would utterly fail.
Considering its probable uses for many practical purposes, and its
possible introduction into laboratories for scientific research, a few
additional remarks as to the construction of such a coil will perhaps
not be found superfluous.
It is, of course, absolutely necessary to employ in such a coil wires
provided with the best insulation.
Good coils may be produced by employing wires covered with several
layers of cotton, boiling the coil a long time in pure wax, and
cooling under moderate pressure. The advantage of such a coil is that
it can be easily handled, but it cannot probably give as satisfactory
results as a coil immersed in pure oil. Besides, it seems that the
presence of a large body of wax affects the coil disadvantageously,
whereas this does not seem to be the case with oil. Perhaps it is
because the dielectric losses in the liquid are smaller.
I have tried at first silk and cotton covered wires with oil
immersion, but I have been gradually led to use gutta-percha covered
wires, which proved most satisfactory. Gutta-percha insulation adds,
of course, to the capacity of the coil, and this, especially if the
coil be large, is a great disadvantage when extreme frequencies are
desired; but on the other hand, gutta-percha will withstand much more
than an equal thickness of oil, and this advantage should be secured
at any price. Once the coil has been immersed, it should never be
taken out of the oil for more than a few hours, else the gutta-percha
will crack up and the coil will not be worth half as much as before.
Gutta-percha is probably slowly attacked by the oil, but after an
immersion of eight to nine months I have found no ill effects.
I have obtained in commerce two kinds of gutta-percha wire: in one the
insulation sticks tightly to the metal, in the other it does not.
Unless a special method is followed to expel all air, it is much safer
to use the first kind. I wind the coil within an oil tank so that all
interstices are filled up with the oil. Between the layers I use cloth
boiled out thoroughly in oil, calculating the thickness according to
the difference of potential between the turns. There seems not to be a
very great difference whatever kind of oil is used; I use paraffine or
linseed oil.
To exclude more perfectly the air, an excellent way to proceed, and
easily practicable with small coils, is the following: Construct a box
of hard wood 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, become, at least with higher frequencies, so easy that
they could be hardly called engineering feats. With oil insulation
and alternate current motors transmissions of power can be effected
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 purposes 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
20, 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; this
allowing the use of a much bigger 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.