if it were turned through 90°, the connections would be interrupted, as the contact-springs ff' would bear against the uncovered portions of the insulating cylinder. The milled head is of course insulated from the axle-ends so as to protect the operator.

617. 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 from an 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

D

convolutions of the secondary coil, and on the suddenness

C of the interruptions of the primary current. The quantity A of electricity which passes through the secondary coil depends on the product of the number of convolutions by the number of tubes of force which cut through them (§ 613 F), and is the same whether the cessation be sudden or gradual; but the electro-motive force varies inversely as the time occupied.

cop

Fig. 549. Statham's Fuse.

The discharges from a Ruhmkorff's coil become more violent and detonating if the two electrodes are con nected respectively with the two coatings of a Leyden jar or other condenser. An apparatus consisting of numerous sheets of tin-foil separated by oiled silk (alternate sheets of foil being connected) is frequently employed for this purpose, and is placed beneath the instrument so as to be out of sight.

Induction coils are often used for firing mines, by means of Statham's fuse, which is represented in the annexed figure (Fig. 549). Two copper wires covered with gutta-percha have their ends sepa

DISCHARGE IN RAREFIED GASES.

765

rated 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.

618. Discharge in Rarefied Gases.-When the ends of the secondary coil are connected with the electrodes of the electric egg (Fig. 550), which has first been exhausted as completely as possible by the air-pump, 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 darkIf other gases are used instead of air, the tints change, but there is always a decided difference of tint between the positive and negative extremities. By the aid of the commutator it is easy to reverse the current, and thus produce at pleasure an interchange of the appearances presented by the two terminals.

ness.

Fig. 550. -Electric Egg.

If, before exhausting, we introduce into the egg a little alcohol, turpentine, or other volatile liquid, the light presents a series of bright bands alternating with dark spaces. Plate II. Fig. 1 represents these stratifications as seen in vapour of alcohol.

The phenomenon of stratification is seen to more advantage in long tubes than in the electric egg; and the presence of alcoholic or other vapour may be dispensed with if the exhaustion be carried sufficiently far, as in the tubes constructed by Geissler of Bonn, which contain various gases very highly rarefied, and have platinum wires sealed into their extremities to serve as electrodes. Four such tubes are represented in Plate II. Certain substances, such as uranium glass, and solution of sulphate of quinine, become luminous in the presence of the electric light, and are called fluores

cent. Such substances are often introduced into Geissler's tubes, for the sake of the brilliant effects which they produce.

+

619. Action of Magnets on Currents in Rarefied Gases.-The luminous discharges in Geissler's tubes are, like the voltaic arc, veritable currents. They are capable of deflecting a magnetized needle, and are themselves acted on by magnets, as in the following experiment. A soft-iron rod (Fig. 551) is fitted in the interior of a glass vessel from which the air can be exhausted, and is coated with an insulating substance to prevent discharge between it and a metallic ring which surrounds it near its lower end. When the terminals of a battery are connected, one with this ring, and the other with the upper end of the ap paratus, a luminous sheaf extends from the summit towards the wire ring, and surrounds the soft iron. If, while things are in this condition, we place beneath the apparatus one pole either of a permanent magnet or an electro-magnet, the soft-iron rod is magnetized, and the luminous streaks immediately begin to revolve round it, the direction of rotation being always in accordance with the rule of § 531 B.

Discharge.

620. Magneto-electric Machines.-Faraday's discovery of the induction of currents by magnets, was speedily utilized in the construction of magneto-electric machines, which, without a battery, and with no other stimulus than that afforded by the presence of a permanent magnet, enable the operator, by the expenditure of mechanical work, to obtain powerful electrical effects. The first machine of this kind was constructed in 1833 by Pixii. A magnet A was made to revolve close to a double coil B B', in which a current was thus generated. The construction was improved by Saxton, and afterwards by Clarke, who made the magnet fixed, and caused the coil, which is much lighter, to rotate in front of it. Clarke's machine is extremely well known, being found in nearly all collections of physical apparatus.

621. Clarke's Machine. In this machine there is a compound

MAGNETO-ELECTRIC MACHINES.

767

horse-shoe magnet fixed to a vertical support. Close in front of the

ries a pinion. By means of an endless chain passing over this pinion, and over a large wheel to which a handle is attached, the

pinion, and with it the coils, can be made to revolve rapidly.

The ends of the wire which forms the two coils are connected respectively with the two metallic pieces E, E' (Fig. 554), which are mounted on the axis, but insulated from it and from each other.

Let us now examine the formation of the currents. The two iron cores, with their connecting iron plate, may be regarded as a temporary horseshoe magnet, whose poles are always of opposite name to those of the steel magnet which are respectively nearest to them. The intensity of magnetization is greatest when the

soft-iron magnet is horizontal, vanishes when it is vertical, and in passing through the vertical position undergoes reversal. If we call one direction of magnetization positive and the opposite direction negative, the strongest positive magnetization corresponds to one of the two horizontal positions, and the strongest negative to the other, the two positions differing by 180°. While the magnet, then, is revolving from one horizontal position to the other, its magnetization is changing from the strongest positive to the strongest negative, and this change produces a current in one definite direction in the surrounding coil. During the next half-revolution the magnetization is again gradually reversed, and an opposite current is generated in the coil. If we examine the direction of the currents due to the cutting across of the lines of force of the permanent magnet by the convolutions of the coil, we shall find that they concur with those due to the action of the cores. The current in the coils circulates in one direction as long as the electro-magnet is moving from one horizontal position to the other, and changes its direction at the instant when the cores come opposite the poles of the steel magnet.

By the aid of the commutator represented in Fig. 554, the currents may be made to pass always in the same direction through an external circuit. r and r'are two contact-springs bearing against the two metal pieces E, E', which are the terminals of the coil. At the instant when the current in the coil is reversed, these springs