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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.

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

Fig. 539.-Electric Egg.

818. Experiments of Gassiot and De La Rue.-By means of a battery of some thousands of cells, discharge in rarefied gases can be obtained without the use of an induction-coil, and with the advantage of greater steadiness. This has been done by Mr. Gassiot, and on a larger scale by Mr. De La Rue. The stratifications are still observed, and appear absolutely fixed in position to the naked eye. When examined by a revolving mirror they are found to exhibit the appearance of a rapid succession of discharges.

Mr. De La Rue's battery consists of 11,000 small chloride of silver cells; and when all the cells are used, a steady stream of fire passes between the terminals as soon as they are brought within about half an inch of each other, in air at atmospheric pressure.

819. Action of Magnets on Currents in Rarefied Gases.-The luminous discharges in Geissler's tubes exhibit the properties of 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. 540) 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 apparatus, 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 § 712.

Fig. 540.-Action of Magnets on the
Discharge.

820. 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 BB' (Fig. 541), 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.

821. Clarke's Machine-In this machine there is a compound horse-shoe magnet fixed to a vertical support. Close in front of the

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magnet, near its poles, are two connected coils t, t', each containing a

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. 543), which are mounted on

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. 543, the currents may be made to pass always in the same direction through an external circuit. r and rare 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 are in contact with intermediate insulating pieces which separate the metallic pieces E, E'. When the current in the coil is in one direc

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tion (say from E to E'), r is in contact with E, and r' with E'. When the current in the coil is in the opposite direction (E' to E), r is in contact with E', and r' with E; thus in each case r is the positive and r' the negative spring, and the current will be from r to r' in an external connecting wire. 00, O'O', are metallic pieces insulated from each other, and connected with the springs rr' respectively. Binding-screws are provided for attaching wires through which the current is to be passed.

With this machine water can be decomposed, wire heated to redness, or soft iron magnetized; but these effects are usually on a small scale on account of the small dimensions of the machine.

For giving shocks, two wires furnished with metallic handles are attached to the binding-screws, and a third spring is employed which puts the terminals EE' in direct connection with each other twice in each revolution, by making contact with two plates q. When these contacts cease, the current is greatly diminished by having to pass through the body of the person holding the handles, and the extracurrent thus induced gives the shock. To obtain the strongest effect, the hands should be moistened with acidulated water before grasping the handles.

822. Magneto-electric Machines for Lighthouses.-Very powerful effects can be obtained from magneto-electric machines of large size driven rapidly. Such machines were first suggested by Professor Nollet of Brussels; and they have been constructed by Holmes of London and the Compagnie l'Alliance of Paris. It is by means of these machines that the electric light is maintained in several lighthouses; they have also been employed to some extent in electro-metallurgy. Fig. 544 represents the pattern adopted by the French company. It has eight rows of compound horse-shoe magnets fixed symmetrically round a cast-iron frame. They are so arranged that opposite poles always succeed each other, both in each row and in each circular set. There are seven of these circular sets, with of course six intervening spaces. Six bronze wheels, mounted on one central axis, revolve in these intervals, the axis being driven by steam-power transmitted by a pulley and belt. The speed of rotation is usually about 350 revolutions of the axis per minute. Each of the six bronze wheels carries at its circumference sixteen coils, corresponding to the number of poles in each circular set. The core of each coil is a cleft tube of soft iron, this form having been found peculiarly favourable to rapid demagnetization.