coatings of a condenser of the kind described in § 628, stowed away in the flat wooden stand which forms the base of the instrument. It serves to mitigate the intensity of the extra-current in the primary coil at breaking circuit, some of the electricity rushing into the condenser instead of assisting to produce a spark at the place where the break occurs.

The successive makes and breaks are effected automatically in various ways. In small instruments the arrangement adopted is usually the same as that of the vibrating alarum described in § 840,

but for large instruments Foucault's contact-breaker is preferred. It is represented in its place in Fig. 536.

The wires from the battery are attached at b and b'. The current, entering for example at b, passes to the commutator C, and thence, through a brass bar let into the table, to the end ƒ of the primary coil. Having traversed this coil, it comes out at f', and is conducted to a vertical pillar, carrying at its upper end a spring, to which the transverse lever L is attached. One end of the lever carries a point which just dips in the mercury of the vessel M, the bottom of which is metallic, and is in communication with b'. The other end of the lever carries a small armature of soft iron just above the end of the

core.

When the current passes, the core becomes magnetized and attracts this armature, thus lifting the point at the other end of the lever out of the mercury and breaking circuit. The core being thus

A

demagnetized, the elasticity of the spring releases the armature, and the point again dips in the mercury, and completes the circuit. thin layer of absolute alcohol is usually poured on the surface of the mercury, and serves, by its eminent non-conducting power, to make the interruptions and renewals of the current more sudden.

The commutator C serves to stop the current from passing or to make it pass in either direction, at pleasure. It is represented in end view and bird's-eye view in the two parts of Fig. 537. There is a cylinder of insulating material, turning by means of metallic axle-ends on metallic supports connected with the two ends of the primary coil. One axle-end is permanently connected by means of the screw 9 with the brass plate C on the surface of the cylinder, and the other axle-end is in like manner connected with the plate C' diametrically opposite to C. The contact-springs ff" are in permanent connection with the two binding-screws A A' which receive the wires from the battery. When presses against C, and f against C', as shown in the figure, A is connected with one end of the primary coil and A' with the other; and when the commutator is turned (by its milled head) through 180°, these connections will be reversed. If it is turned through 90°, the connections will be interrupted, as the contact springs will bear against insulating portions of the cylinder. The milled head is, of course, insulated from the axle-ends so as to protect the operator.

816. Spark from Induction-coil.-When the ends of the secondary coil are connected, currents traverse it alternately in opposite directions, as the primary circuit is made and broken. These opposite currents convey equal quantities of electricity, and if they are employed for decomposing water in a voltameter, the same proportions of oxygen and hydrogen are collected at both electrodes. If, however, the ends are disconnected, so that only disruptive discharge can

occur between them, the inverse current, on account of its lower electro-motive force, is unable to overcome the intervening resistance, and only the direct current passes (that is, the current produced by breaking the primary circuit). The sparks are usually from 1 inch to about 18 inches long, according to the size and power of the apparatus, and exhibit effects comparable to those obtained by electrical machines. A Leyden battery may be charged, glass pierced, or combustible bodies inflamed.

The great electro-motive force of the induced current, which enables it to produce these striking effects, depends on the great number of convolutions of the secondary coil, and on the suddenness of the interruptions of the primary current. The average electro-motive force is the product of the number of convolutions by the number

of tubes of force which cut through them, divided by the time occupied (§ 806).

The discharges from a Ruhmkorff's coil become more violent and detonating if the two electrodes of the secondary coil are connected respectively with the two coatings of a Leyden jar, but the length of the spark is very much diminished.

Induction-coils are often used for firing mines, by means of Statham's fuse, which is represented in the annexed figure (Fig. 538). Two copper wires covered with gutta-percha have their ends separated by a space of a few millimetres, and inclosed in a little cylinder of gutta-percha containing sulphuret of copper. This, again, is inclosed in a cartridge, CD, which is filled up with gunpowder. The two wires are connected with the two ends of the secondary coil, and when the instrument is set in action, sparks pass between the ends A, B, heating the sulphuret of copper to redness, and exploding the powder.

Fig. 538.

Statham's Fuse.

817. Discharge in Rarefied Gases.-When the ends of the secondary coil are connected with the electrodes of the electric egg (Fig. 539), which has first been exhausted as completely as possible by the airpump, a luminous sheaf, of purple colour, is seen extending from the positive ball to within a little distance of the negative ball. The latter is surrounded by a bluish glow. The blue and purple lights are separated by a small interval of darkness. If other gases are used instead of air, the tints change, but there is always a decided

ELECTRIC DISCHARGE IN RAREFIED GASES.

1. Discharge in Vapour of Alcohol. (2.3.4.5. Geissler's Tubes enclosing rarefied Cases.) 2. Shows the Fluorescence of Sulphuret of Calcium. 4. Fluorescence of Uranium glass. 5. Fluorescence of Sulphuret of Strontium. 6. Fluorescence of Uranium glass, and of Sulphate of "Quinine.