Bertsch's Rotatory Electrophorus.

705

warm flannel, and is thereby rendered highly electric negatively. The metal plate, being then placed upon it, has positive electricity induced on the face next the cake, and negative on the farther face. If now with the finger we touch the upper surface, the negative on the upper surface escapes to the earth, while the positive remains on the plate; and on lifting the plate with its charge away from the influence of the cake, we obtain a spark of greater or less intensity; and by simply repeating the operation of placing the cover on the cake, touching the cover, and then lifting it, we may obtain any number of such sparks. The spark may be strong enough to light a gas jet, and to serve many useful electrical purposes; and the operation may be repeated any number of times for days or even months, without any necessity for renewing the friction with the catskin.

A cake of vulcanite or ebonite, it may be remarked, forms a very convenient substitute for the more brittle compound resinous cake above described.

943. Rotatory Electrophorus.-Within recent years there have been devised various methods of employing the continuous electrophorus principle for the production of electricity in quantity, and without the labour of touching with the finger and lifting the cover for each little electrical charge.

A form of revolving electrophorus, the invention of M. Bertsch, is shown in fig. 256, where the insulating supports are omitted in order to render the principle of its

action more clear. It consists of a vulcanite disc, v, about 20 inches in diameter,

N

I

which can be rapidly rotated by an arrangement of multi- PO plying wheels, H. A sector of vulcanite, I, acts as the inductor, or inducing source of clectricity. Opposite to I, and on the other side of the vulcanite plate, V, is a metallic comb or rake, C, which communicates with the conductor, N; at the extremity of the same diameter of the plate is a second metal comb, c', which communicates with the other conductor, P. If we excite the inducing piece, I, by striking with a catskin or otherwise, and bring it near to the

Fig. 256.

706

Holtz's Electric Machine.

plate, while we rapidly rotate the latter, it is found that a series of brilliant electrical sparks pass between P and N, the two poles of the machine; and in dry frosty weather we may thus have an indefinite source of electricity by merely rotating the handle, H.

The theory of the electric action is rather complicated, but it may be roughly given in this way:-The negative electricity in I induces positive on the face of the plate, V, next itself, and negative on the opposite face next the comb. The latter combines readily with the positive in the conducting system, N C, owing to the effect of the pointed teeth of c. (See Art. 938.) Thus N C remains negatively charged; and the part of the vulcanite disc opposite to it positively charged. As the disc rotates, the part of the plate positively charged comes opposite to the comb-rake, c', of the conducting system, PC', and in the same way as before discharges the negative electricity of this system, charging P positively; and passing on in the neutral condition as at first towards the inductor, I, where the same effects recur. With a rapid rotation of the disc, and a dry atmosphere, there may in this way be accumulated a sufficient quantity and strength of opposite electrical charges in the two poles, P and N, of the machine, to dart through the distance of a few inches. The whole forms an exceedingly simple and elegant substitute for the old cylinder or plate machines. 944. Holtz's Induction Machine.—Somewhat analogous in prin

A B

ciple to the machine just described is that invented by M. Holtz of Berlin, in 1865, which has become quite a favourite with electricians. The figure gives a simple plan of the machine; the omitted details of insulation will be readily understood. It consists of two glass plates, A and B (fig. 257), of which the former can be rapidly rotated by the multiplying wheels, w, w'; while B B remains stationary. The centre of the fixed wheel is pierced with an opening to allow the axle of A to pass through; and two other openings or windows are made in a horizontal diameter of B B, opposite to I, I', which are pieces of paper or tinfoil pasted on the disc, and having tongues projecting into the windows. On the other side of the rotating disc, A, are

Fig. 257.

metallic combs or rakes, C, C', which communicate with the conducting systems, N and P. The action of the machine may be summarily explained as follows:-One inductor, I, is charged negatively by contact with a piece of excited vulcanite; I induces positive electricity on the face of A next to it, and negative on the opposite face; the latter discharges the positive of the conducting system, N C, and leaves the latter negatively charged. The wheel turns round towards C', charged positively, and electrifies I' also positively, while, at the same instant, it draws the negative electricity of the conducting system, P C', through the teeth of the comb, c', rendering P positively electrical. Thus the sector of the plate which left C with positive, leaves c' with negative electricity, and returning to C so charged, at once heightens the negative charge of the inductor, I, and of the pole, N. The accumulation of opposite electricities in N and P, as well as in I and I', in this way proceeds at a compound rate; and a torrent of very powerful sparks will in course pass between the two poles of the machine.

In dry, frosty weather this apparatus works most admirably, and is infinitely superior to the older machines, only an exceeding sensitiveness to moisture renders it a much more troublesome servant.

Many other electrical accumulators have been constructed on the same principle as those above mentioned, by Sir W. Thomson, Mr. Varley, and others; but the above will serve as types of the whole class.

945. The Leyden Jar.—The inductive condensation or cumulation of electricity by means of what is now known as the Leyden jar, was discovered by accident in the year 1748, to the great surprise of those who witnessed it, and of the scientific world generally.

An experimenter at Leyden, in Holland, with a view to ascertain the effect of electrifying water, placed in a phial containing water a short brass rod, and then held the end of the rod in contact with the charged conductor of an electrical machine. Having charged the water fully, as he supposed, he removed it from the contact, and then applying the other hand to withdraw the wire from the phial, he instantly received a violent shock through his arms and body, which caused him to drop the phial, and himself to sink benumbed to the floor. He believed that he had narrowly escaped death; and he afterwards said that he would not take another such shock if the empire of Germany were offered as the bribe.

708

a

Electrical Accumulation.

It was soon discovered, however, that the charge was not in the water, but connected with the surfaces of the glass, the water serving merely as the conducting medium which joined together in action the several parts of the internal surface of the non-conducting glass; for the same effect was produced by substituting for the water a leaf of tinfoil, which could similarly cover the surface of the glass. And if both the internal and external surfaces of a bottle or jar were covered with tinfoil, to within a short distance of the top, the jar could then receive a c very powerful charge indeed. The adjoining figure (258) shows the common form of jar so covered, and known as the Leyden Far. A brass rod with a knob at its top passes through a wooden stopper into the jar, and has a piece of chain hanging from its bottom in contact with the metallic lining, to establish communication. In order to charge such a jar we bring the knob, a, into communication with some source of electricity, such as the prime conductor of any of the electric machines already described; while we connect the outside coating with the earth either by holding the jar in the hand, or setti ig it on a table and fastening a long chain round it. The mode of action may be conceived to be this. While the inner coating of the jar is charged with positive electricity it induces negative on the face of the outside coating next itself, and positive on the outer face; and unless the latter be allowed to escape, a limit to the inductive action of the inner coating is shortly reached.

Fig. 258.

946. This relation between the electrical states of the two sides of a moderate thickness of glass, or other non-conductor of elec

tricity, is shown by the fact that if a Leyden jar resting on a support of glass be charged by distinct sparks from a prime conductor-as the jar, B, from the conductor, A (fig. 259)—then for every spark

The Electric Battery.

709

which enters it a corresponding spark is driven from the outside to any conductor placed near, as C, for instance, which is another jar, and if a row or series of such insulated jars stand as here represented, the charging of the first one, B, will charge all the others to the same degree; and if the outer coating of the last be connected with the earth, E, then the whole three jars may be simultaneously charged very strongly.

It is, however, the glass that is the chief seat of the electrical accumulation. For if a common glass tumbler, a (fig. 260), be set into a metallic vessel, b, which exactly fits and covers the

α

7

Fig. 260.

lower part of its external surface, and if it then receive into itself a smaller tin vessel, c, with a handle of glass which closely fits the internal surface, the combination of the three, a, may be strongly electrified and discharged like a coated Leyden jar; but if, after being electrified, the smaller tin cover is lifted out by its glass handle, and the external tin cover is allowed to fall off, these tins are found to be in a natural state, and the whole electrical charge remains on the surfaces of the glass. The charge so left can be made to produce its usual effects by replacing the tins.

947. The Electric Battery.-The discovery of the nature of the Leyden jar gave an increased power of accumulating electricity, which changed the character of clectrical proceedings. By connecting the interiors of many jars, and also the exteriors, to cause them to act like one larger jar, as represented in fig. 261, effects may be produced similar to those of the natural lightning of

Fig. 261.

a thunder-storm. This combination is possible to any extent, and is called an Electric Battery.

948. The adjoining cut represents what is called a General Discharger, by means of which an experimenter can, without danger to himself, send the charge of the most powerful battery through any substance exposed in its way. There are two branches, a c and