repelled the positive or similar fluid to the farthest point, p. This is what we should expect from the law that unlike electricities attract and like electricities repel each other.

It would seem, however, that there is here an "action at a distance," were it not for the presence of the air between the two bodies; and Faraday has shown that every molecule of air between the two bodies is acted on in the same way as the cylinder itself, and has become "polarised" like it--that is to say, has its side nearest P showing a negative charge, and its side nearest the cylinder a positive charge, according to the imaginary Fig. 12, where P and N are the two bodies,

and the intervening circles are supposed to be the air molecules. Should the mutual attraction between P and N become too great for the air molecules to bear, the molecular bridge between them will break down under the stress, and the electricities will rush together through the air with a cracking sound and flash of light. This is the action which equally takes place when a spark is drawn from the prime conductor of an electric machine, or when a flash of lightning passes between an electrified cloud moving over the surface of the earth and inducing opposite electricity on the fields, trees, and steeples below. As electricity tends to discharge from points, it is generally through some prominent object in the landscape that the lightning discharge takes place, and hence the necessity of having all high buildings protected.

We have said that this principle of induction is utilised in the construction of machines for generating electricity-notably in the Holtz frictional machine. And although we need not enter into the details of that complex apparatus, it will be requisite to explain how the principle is applied. Returning, then, to Fig. 11, it will readily be understood that if the charged body, P, be withdrawn from the cylinder, the two separated electricities will at once recombine, and the transient separation will exist no more. But if, while p is near the cylinder, we touch the remote end p, and thus take away the positive charge thereon, then on withdrawing our finger again we shall still leave the negative charge at the end n; and on removing P, this negative charge, having no longer an equal positive charge to combine with, will remain upon the cylinder as a free and permanent charge. In this way, then, can induction be made to generate new charges of electricity.

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Not only, however, does a body with a fixed charge of electricity induce an opposite charge in another body standing near it, but a current of electricity flowing in a wire induces an opposite current in another wire close by. This is the greatest discovery of the immortal Faraday, and upon it are founded all the "induction coils" used by medical men for giving shocks to patients, and the modern dynamo-electric machines for generating the currents to feed electric lights and drive electric motors. In the year 1831 Faraday found that whenever an electric current is suddenly sent along a wire w, Fig. 13, as shown by the arrow, it instantly excites an opposite current in a second wire, w' held parallel to the first. This induced current,

however, is only momentary, and is evidently due to the first passage of the primary current through its wire. Though the primary current is kept flowing, the induced, or secondary current disappears; but when

W

w'

FIG. 13.

the primary current is suddenly stopped the induced one reappears, but flowing in a direction contrary to that which it followed before. In fact, the disturbance caused by starting and stopping the current in the primary wire has the effect of setting up a contrary disturbance in the neighbouring wire.

The apparatus known as the "Induction Coil," which is useful for giving a constant stream of sparks, is based upon this discovery. Its action will be readily understood from the form shown in Fig. 14, which we have chosen because of its simplicity. There B is a coil wound of a short length of stout copper wire insulated with silk and varnish; and B' is a coil. wound of a long length of fine copper wire similarly insulated. These two coils are shown apart, but B' can be slid over B so as to bring one within the other. When the current from a voltaic battery is started in the coil в a momentary current is induced in B'; flowing in a reverse direction to the current in B; and when the current in B is stopped, another momentary current is induced in B' flowing in a contrary direction. to the current induced in it when the current in B was started. The coil в is called the "primary," or induc

ing circuit; while B' is the "secondary" or induced circuit. If, then, the poles of a voltaic battery are connected to the ends of the coil в through a device for making and breaking the connection between them, every time the connection is made a pulse or jet of current will traverse the coil B' in a contrary direction. to the flow of the current in B; and every time the connection between the battery and в is broken, a similar jet or current will traverse the coil B' in the

same direction as that of the current which had just been flowing in B. Consequently, by rapidly making and breaking the connection between B and the battery, a rapid succession of contrary jets or pulses of electricity is set up in B', and as these jets are of a much more intense nature than the current in B, if we keep the two ends of the coil B' almost but not quite touching, we shall get a stream of sparks between the points of the wire. The sparks will be strongest when в is

inside B', but their strength is only moderated by separating the coil further apart. The apparatus shown is known as the "sledge" coil, because B' can be slid along a graduated scale н н, so as to moderate the sparks to different degrees of intensity.

The interruption of the current in the "primary" coil is effected automatically by means of an electromagnetic contact breaker shown at D and E. The wires from the voltaic battery come to binding-screws a a', one of which A' is connected to one end of the coil B through the coils of a double electro-magnet D and the wire ; while the other terminal A is connected to the other end of the coil в through the standard a, the arm E, and the screw v. Now the current flows from one pole of the battery to A, then up the standard a, along the arm E, and up the screw v with which it is in contact, through the coil в, and back to the other pole of the battery through the coils D of the electro-magnet. But in traversing the coils D it magnetises the iron core in these coils, and thus attracts a piece of soft iron or "armature" at the end of E, and pulls the latter down, thereby breaking the contact between E and v. This interrupts the current in the coil û, and demagnetises the poles D. The arm E therefore springs back into contact with v, and the current is again established in B, only to be interrupted and established as before. This automatic making and breaking of the current in B goes on as long as the poles of the battery are connected to the terminals A A', and hence a corresponding succession of alternating currents is maintained in B'. These result in a continual stream of sparks between the terminals of that coil.

To increase the inductive effect, a bundle of iron