RUHMKORFF'S, HEARDER'S, AND BENTLEY'S COIL APPARATUS.
In the course of the popular articles on frictional and voltaic
electricity, it has already been mentioned that whilst the _intensity
effects_--such as the capability of the spark to pass through a certain
thickness of air, or the production of the peculiar physiological effect
of the shock--belong especially to the phenomena of frictional
electricity, they are not apparent with the _quantity effects_, such as
may be produced by an ordinary voltaic battery, unless the latter
consists of an immense number of elements, such as the famous water
battery of the late respected Mr. Crosse, which consisted of two
thousand five hundred pairs of copper and zinc cylinders, well insulated
on glass stands, and protected from dust and light. If, however, the
feeble intensity current of voltaic electricity, from four or five
elements, is permitted to pass into a coil of a peculiar construction,
fitted with a condenser, and manufactured either by Ruhmkorff of Paris,
or Mr. Hearder of Plymouth, then the most remarkable effects are
producible, which have created quite a new and distinct series of
phenomena, and further established in the most satisfactory manner the
connexion between the electricities derived from _friction_ and
_chemical action_.
The construction of these coils does not differ very materially, and
great merit is due to Messrs. Ruhmkorff, Hearder, and Bentley, who have
separately and independently worked out the construction of the most
formidable machines of this class. In a letter to the author Mr. Bentley
says:--
"I commence the formation of my coil by using as an axis an iron tube
ten inches long and half an inch diameter; around this is placed a
considerable number of insulated iron wires the same length as the tube,
and sufficiently numerous to form a bundle one inch and three quarters
diameter. This core is wrapped carefully in eight or nine layers of
waxed silk, the necessity of which will be obvious presently.
"My primary helix, which is formed of thirty yards of No. 14
cotton-covered copper wire, is wound carefully on this core, and
consists of two layers, each layer being carefully insulated one from
the other by waxed silk, for I find that if a wet string or fine
platinum wire be connected with the two ends of the primary wires of an
induction coil in action, there is scarcely an indication of an induced
current to be obtained from the secondary wire. That this is not owing
to any decrease of magnetic power is proved by testing the iron core
before and after the experiment, but is simply owing to the central
magnet or coil exerting the whole of its inductive powers upon the
nearest closed circuit; it therefore follows that if the two layers of
primary wire are connected by the cotton covering becoming moist, the
whole of the [Page 231] induced current will take this path instead of
traversing the secondary wire.
"Before describing my secondary wire I must again call attention to the
important fact that the magnetism of the iron exerts its inductive power
upon the nearest conducting medium; and I have constructed an instrument
to demonstrate this fact. It consists simply of an ordinary coil, giving
the third of an inch spark, but having the four inner layers of
secondary wire brought out separately. Now, I find that when I keep the
ends of this wire separate I obtain nearly the third of an inch spark,
but when I connect them metallically I can obtain no intensity spark
whatever from the seventeen coils which surround them.
"It follows from this that before winding the secondary wire the
striking distance of a single layer must be ascertained, and I find that
with my coil I can get a spark one-tenth of an inch long from one coil
of wire, and sufficiently intense to penetrate with facility six layers
of waxed silk.
"Waxed silk is therefore unsuited for the insulation of large coils, and
I find, after numerous experiments, that there is no substance so fitted
for the purpose as gutta-percha tissue, and I use five layers of this
substance to each layer of wire.
"The secondary helix then consists of three thousand yards of No. 35
silk-covered copper wire, and is insulated in the manner described
above; but as I do not use cheeks to my coil it assumes the form of a
cylinder having rounded ends.
"For the protection of this instrument I place it in a mahogany box of
the proper size, and it is supported and retained in its position by an
iron rod, which is thrust through the hollow axis of the core and the
two ends of the box, leaving half an inch of the iron projecting to work
the contact breaker, which is fixed to one end of the box, while the two
ends of the secondary wire are brought out of the other through gutta
percha tubes.
"The condenser is contained in a separate box, and is formed of one
hundred and twenty sheets of tinfoil between double that number of
sheets of varnished paper, the alternate sides of the foil being brought
out and connected to appropriate binding screws.
"This condenser forms a convenient stand for the coil, and can be used
for many interesting experiments."
The shock which the condenser gives to the system depends in a great
measure on the size of the coatings. The primary wire alone does not
produce any physiological results, or at least very feeble ones. Mr.
Hearder's coil is wound on a bobbin six inches in length, and four
inches and a half thick, and includes three thousand yards of covered
wire (No. 35). The iron core consists of a bundle of small wires capped
with solid ends, and the sparks obtained from it were five-eighths of an
inch in air when the primary coil was excited by four pairs of Grove's
series; and when connected with the Leyden jar, the most vigorous and
brilliant results were produced. The condenser is made of cartridge
paper, coated in the proper manner with tinfoil. The secondary [Page
232] coil is quite independent of the primary one, which is laid on in
different lengths, so that the coil can be adjusted to any battery
power, whether for quantity or intensity.
For the successful exhibition of the capabilities of the machine, it is
required to perform the experiments in a darkened room. (Fig. 221.)
[Illustration: Fig. 221. Ruhmkorff's apparatus. A B. The coil,
containing more than a mile of insulated wire. The stand it rests upon,
and with which it is in communication, contains the _condenser_.]
In using this apparatus, eight pairs of Grove's battery will be quite
sufficient to produce the effects, and the greatest care must be taken
to avoid the shock, which is most severe and painful, and might do a
great deal of harm to a weakly, sensitive, and nervous person. To avoid
any accidents of this kind, the convenient arrangement at one end shown
in Fig. 222 must be carefully attended to, and when manipulating with
any part of the apparatus, if the battery is attached, the contact
should first be broken by bringing the ivory (the non-conducting) part
of the cylinder A (Fig. 222) in communication with the conductors, B B,
where the wires from the battery are attached.
[Illustration: Fig. 222. One end of Ruhmkorff's coil. B B. Connexion to
receive the battery wires. A is the cylinder, one half of which is ivory
and the other metal. In this position no shock can be received, because
the electricity is cut off by the ivory from the coil.]
_First Experiment._
It is at the other extremity of the coil that the experiments are
performed; for instance, if an exhausted globe is connected with the
pillars B B (Fig. 223), and the connexion made with the battery, a
beautiful faint blue light is apparent on one of the knobs and wires,
and by reversing the current the light appears on the other knob and
wire. [Page 233] This effect is supposed to resemble some of those
magnificent streaks and undulations of coloured light called the Aurora
Borealis; and, if the globe is removed from the foot, and screwed on to
the air-pump plate, and a little alcohol, ether, naphtha, or turpentine
placed on wool or tow is held to the air-pump screw, where the air
usually rushes in, and the cock turned, so that the vacuum is destroyed,
a quantity of the vapour will necessarily fill the globe; and if this is
once more exhausted, it presents a different appearance, being full of
coloured light (varying according to the spirit employed) but stratified
and of a circular form. (Fig. 223.)
[Illustration: Fig. 223. End of coil where the experiments are
performed. B B. Connecting screws and wires passing to the exhausted
globe, C. The screws are supported on insulating glass pillars, P P.]
_Second Experiment._
The appearance of these bands of light is modified by the nature of the
glass tubes employed, and the subject has been carefully investigated by
Mr. Gassiott. At the last meeting of the British Association at
Aberdeen, Dr. Robinson made various experiments, arranged by Mr. Ladd,
for the purpose of showing the connexion between these miniature effects
of bands of light in tubes containing various gases, and the phenomena
of the Aurora Borealis. The title of the discourse, which was specially
delivered in the Music Hall by the learned Doctor, was "On Electrical
Discharges in Highly-rarefied Media," and it was illustrated by
experiments prepared by Mr. Gassiott and Mr. Ladd.
The kind of tubes employed may be understood from the next figure. They
are made in Germany, and by approaching a powerful magnet to [Page 234]
the outside of any of the glass tubes whilst the bands of light are
being produced, the most remarkable modifications of them are obtained.
Mr. Ladd has mounted one of these tubes in a rotatory arrangement
similar to that described at page 186. When connected with the coil and
battery, it furnishes one of the most lovely "electric fire-wheels" that
can possibly be described. (Fig. 224.) Mr. Grove placed a piece of
carefully-dried phosphorus in a little metallic cup, and covered it with
a jar having a cap and wire. On removing the air from the receiver, and
passing the current of electricity through it from the Ruhmkorff coil,
he obtained a light completely stratified, and blended transversely with
straight but vibrating dark bands.
[Illustration: Fig. 224. A, B, C, D, E, F. Various tubes of different
kinds of glass, and containing gases and vapours. Each tube has a
platinum wire inserted at both ends, with which the contact is made with
the coil. The tube A contains mercury, which has been boiled in it, and
the air expelled. By moving the conducting wire to G or H, the light
which otherwise passes through the whole of the tubes stops at these
points.]
_Third Experiment._
When two very thin iron wires are arranged in the upright pillars (Fig.
223), and held sufficiently close to each other, as in Fig. 225, light
passes from one to the other. The wire from which the light passes
remains _cold_, the other becomes so _hot_ that it melts into a little
globule of liquid iron, and if paper is held between the wires it
rapidly takes fire. (Fig. 225.)
[Illustration: Fig. 225. Melting of the iron wire.]
[Page 235]
_Fourth Experiment._
Remove the break. Attach two wires to X X (Fig. 226). Hold them so as at
pleasure to complete and interrupt the galvanic circle. Two other wires
are attached at P P, their ends being about three-quarters of an inch
asunder. When the current is closed or broken at A A, a spark passes
between B B. (Fig. 226.)
[Illustration: Fig. 226. The making and breaking of the circuit.]
_Fifth Experiment._
A Leyden jar may be charged and discharged with singular rapidity when
connected with the coil, and the snapping noise is so rapid, that it
produces a continuous sharp sound. (Fig. 227.) If a piece of paper is
held between the ball of the Leyden jar and the wire, it is instantly
perforated, but not set on fire.
[Illustration: Fig. 227. A B. Leyden jar coated with tinfoil, and
standing on any non-conductor, such as gutta percha or the resinous or
glass plate, C.]
[Page 236]
_Sixth Experiment._
When the Leyden jar is coated with spangles of tinfoil, a spark appears
at each break, and the whole jar is lit up with hundreds of brilliant
sparks each time it is charged and discharged, and as this occurs with
amazing rapidity, the light is almost continuous. (No. 1. Fig. 228.) The
larger the Leyden jar, the shorter the spark, and _vice versâ_. By the
employment of a nicely-made screw and inch-scale, the distance between
the discharging points connected with a Leyden jar can be accurately
determined; and Mr. Hearder states that supposing a Leyden jar has one
square foot of charging surface, it will give a spark of one inch in
length, but if a smaller jar is used, with only half a square foot of
charging surface, the spark would be about one inch and a quarter in
length. (Fig. 228.)
[Illustration: Fig. 228.--No. 1. Spangled Leyden jar. No. 2. Hoarder's
apparatus for measuring the length of spark for Leyden jar and coil. P
P. Glass pillars. No. 3. Two best forms of spangles to paste on a Leyden
jar.]
_Seventh Experiment._
The direction and rapidity of the current appear to influence greatly
the heating and fire-giving power of the coil, and the following
experiment, devised by Mr. Hearder, furnishes a curious illustration of
this fact.
When the current passes in the direction of the arrows (Fig. 229), the
[Page 237] platinum wire remains perfectly cool whilst the gunpowder is
fired; and the contrary takes place if the current is reversed--viz.,
the gunpowder does not blow up, but the platinum wire is heated. In the
second experiment, a Leyden jar is included in the circuit. (Fig. 229.)
[Illustration: Fig. 229. A. The coil. B. Hearder's discharger, with thin
platinum wire, P, hanging between the points. C. Another discharger, and
powder going off between the points from the little table. The pillars
of the dischargers are glass. The arrows show the direction of the
current of electricity.]
_Eighth Experiment._
Amongst so many beautiful experiments, it is somewhat difficult to say
which is the most pleasing, but for softness and exquisite colouring,
with the continuous vibrating motion of the flowing current of
electricity, nothing can surpass "the cascade experiment." [This
beautiful experiment is usually termed "Gassiott's Cascade," and is thus
described by that gentleman. Two-thirds of a beaker glass, four inches
deep by two inches, are coated with tinfoil, leaving one inch and a half
of the upper part uncoated. On the plate of an air-pump is placed a
glass plate, and over it the beaker, covering the whole with an
open-mouthed glass receiver, on which is placed a brass plate having a
thick wire passing through a collar of leather; the portion of the wire
within the receiver is covered with a glass tube; one end of the
secondary coil is attached to this wire, and the other to the plate of
the pump. As the vacuum improves the effect is very surprising; at first
a faint clear blue light appears to proceed from the lower part of the
beaker to the plate; this gradually becomes brighter, until by slow
degrees it rises, increasing in brilliancy until it arrives at that part
which is opposite, or on a line with the inner coating, the whole being
intensely illuminated; a discharge then commences, as if the electric
fluid were itself a material body running over.] This result is obtained
by coating the inside of a handsome glass goblet with tinfoil, and
placing it under a jar fitted with a collar of leather and ball, and
arranged in the usual manner on the air-pump. Directly a vacuum is
obtained, the ball is moved down to the inside of the goblet, and the
wires from the coil being attached, a continuous series of streams of
[Page 238] electric light seem to overflow the goblet all round the
edge, and it stands then the very embodiment of the brimming cup of
_fire_, and emblematical of the dangers of the wine-cup. (Fig. 230.)
[Illustration: Fig. 230. Gassiott's Cascade.]
_Ninth Experiment._
If a piece of wood five inches long and half an inch square is placed on
the table of the discharger, and one wire brought on to the top edge and
the other approached to within three inches of it, and touching the
wood, and the space between them moistened with the strongest nitric
acid, a curious effect is visible from the creeping along of the fire,
which gradually carbonizes and burns the wood. (Fig. 231.)
[Illustration: Fig. 231. Burning the piece of wood moistened with the
strongest nitric acid.]
[Page 239]
_Tenth Experiment._
A glass plate wetted with gum, and then sprinkled with various filings
of iron, zinc, lead, copper, &c., produces a very pretty effect of
deflagration as one of the conducting wires is moved over its surface,
the other of course being in contact with the plate. The gum quickly
dries by putting the plate in a moderately-heated oven.
_Eleventh Experiment._
When the continuous discharges from the Leyden jar are made to pass
through the centre of a large lump of crystal of alum, blue vitriol, or
ferroprussiate of potash, &c., the whole of the crystal is beautifully
lighted up during the passage of the electricity from one wire of the
discharger to the other. (Fig. 232.)
[Illustration: Fig. 232. A. The Leyden jar. B. Large lump of alum, with
a hole bored through it in a line with C D. The discharging wires are
brought within three-eighths of an inch of each other, and the whole
crystal is lighted up with the brilliant electric sparks.]
_Twelfth Experiment._
When a piece of paper slightly damped is placed between the wires of the
discharger, the spark is increased to a much greater length, on account
of the conducting power of the water contained in the pores of the
paper; and taking all things into consideration, the author considers he
has witnessed the grandest effects from the coil invented and
constructed by Mr. Hearder, the talented lecturer and electrician of the
West of England.
_Thirteenth Experiment._
Electro-magnetic coil machines have been employed for a very
considerable time in alleviating certain of "the ills which flesh is
heir to," [Page 240] by the administration of shocks. These may be so
regulated as to be hardly perceptible, or may be so powerful that the
pain becomes absolutely intolerable.
These coils are now made self-acting, and consist of two coils of
covered and insulated wire wound round a bundle of soft-iron wires, with
the necessary connecting screws for the voltaic battery. The contact
with the battery is made and broken with great rapidity by a simple form
of break, consisting of a tinned disc of iron held by a spring over the
axis of the bundle of iron wires; and the continual noise of the break,
which is alternately attracted down to the bundle and brought back by
the spring, when the coil is in contact with the battery, demonstrates
(without the pain of taking the shock) when the instrument is in full
working order.
The coil machine is not only useful in a medical point of view, but when
properly arranged offers a good reception to a run-away bellringer, and
is an excellent preventive against illicit attempts at cheap rides by
small boys.
[Illustration: Fig. 233. Boy, _evidently shocked_, behind doctor's
carriage provided with a small coil machine.]
[Page 241]