The Boy's Playbook of Science Part 29

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 pa.s.s 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 _quant.i.ty 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 gla.s.s stands, and protected from dust and light. If, however, the feeble intensity current of voltaic electricity, from four or five elements, is permitted to pa.s.s 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 cla.s.s. 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 a.s.sumes 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 quant.i.ty 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.)

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 k.n.o.bs and wires, and by reversing the current the light appears on the other k.n.o.b 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 c.o.c.k turned, so that the vacuum is destroyed, a quant.i.ty 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.)

[Ill.u.s.tration: Fig. 223. End of coil where the experiments are performed. B B. Connecting screws and wires pa.s.sing to the exhausted globe, C. The screws are supported on insulating gla.s.s pillars, P P.]

_Second Experiment._

The appearance of these bands of light is modified by the nature of the gla.s.s tubes employed, and the subject has been carefully investigated by Mr. Ga.s.siott. At the last meeting of the British a.s.sociation 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 t.i.tle 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 ill.u.s.trated by experiments prepared by Mr. Ga.s.siott 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 gla.s.s 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 pa.s.sing 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.

[Ill.u.s.tration: Fig. 224. A, B, C, D, E, F. Various tubes of different kinds of gla.s.s, 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 pa.s.ses 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 pa.s.ses from one to the other. The wire from which the light pa.s.ses 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.)

[Ill.u.s.tration: Fig. 225. Melting of the iron wire.]

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_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 pa.s.ses between B B. (Fig. 226.)

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 227. A B. Leyden jar coated with tinfoil, and standing on any non-conductor, such as gutta percha or the resinous or gla.s.s 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 versa_. 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.)

[Ill.u.s.tration: Fig. 228.--No. 1. Spangled Leyden jar. No. 2. h.o.a.rder's apparatus for measuring the length of spark for Leyden jar and coil. P P. Gla.s.s 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 ill.u.s.tration of this fact.

When the current pa.s.ses 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.)

[Ill.u.s.tration: 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 gla.s.s. 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 surpa.s.s "the cascade experiment." [This beautiful experiment is usually termed "Ga.s.siott's Cascade," and is thus described by that gentleman. Two-thirds of a beaker gla.s.s, 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 gla.s.s plate, and over it the beaker, covering the whole with an open-mouthed gla.s.s receiver, on which is placed a bra.s.s plate having a thick wire pa.s.sing through a collar of leather; the portion of the wire within the receiver is covered with a gla.s.s 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 gla.s.s 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 br.i.m.m.i.n.g cup of _fire_, and emblematical of the dangers of the wine-cup. (Fig. 230.)