would be indicated by an opposite direction of deflexion in G. In order to distinguish this reverse swing from the fall back to zero, which would ensue merely from the fact that the battery circuit is broken, it is necessary to keep the needle at zero by means of stops placed so as to prevent the former deflexion, but so as to permit of the second deflexion if such there be. Such a deflexion is, in fact, observed; and it demonstrates the existence of the extra current. (The present writer has found that very marked results may be obtained with two Leclanché cells as battery, an ordinary rough astatic galvanometer at G, and a powerful electro-magnet instead of the simple coil B.)

(ii.) If there be in circuit a powerful electro-magnet and a sufficient battery, and if the circuit be broken by a person holding in his moistened hands the two ends of the circuit, a shock will be perceived. The high E. M.F. of the induced extra current is sufficient to drive some current through the human body that has suddenly been interposed in the circuit.

We see, then, that when a circuit is made, the current rises slowly, and research has shown that its rise is oscillatory. It then continues uniform as long as the battery is constant. When the circuit is broken, there is a sudden leap in the magnitude owing to the extra current; and finally it ceases again in an oscillatory manner, much more abruptly than it began. All this is very readily exhibited by means of a curve, the abscissæ measuring time and the ordinates magnitude of current.

§ II. Induced Currents of Higher Orders.

It is found, as might indeed have been predicted, that induced currents will themselves act as inducing currents.

We may arrange a series of coils somewhat as follows. First, a primary A, and round it a secondary B. B may then be in circuit with a coil B' at a distance from A, so that currents induced in B will circulate also in B'; and round B' is placed another coil C upon which B' can act inductively, while A is too remote to have any direct influence. We will use the words 'direct' and 'inverse' when the currents are in the same direction as, or in the opposite direction to, the original current in A respectively.

We find that when the current is made in A, we have an inverse current induced in B and therefore passing in B'. This current, as it rises in strength from zero to a maximum, induces in C an opposed current, which will therefore be direct; and, as it falls again to zero, it induces in C a current in the same direction, which will therefore be inverse.

So, when the current in A is broken, we have a direct current induced in B; and in C, an inverse followed by a direct current.

CHAPTER XXII.

ARAGO'S DISC, RUHMKORFF'S COIL, AND OTHER CASES OF

INDUCTION.

1. Induced Currents (Eddy Currents') in Solid Metallic Masses moving in a Magnetic Field. When a conducting mass moves in a magnetic field currents are induced. Here, as always, the currents are such as to oppose the movement; work must be done on the masses in order to move them, and we have developed in the mass equivalent electric energy, which finally runs down into the form of an equivalent of heat.

Such currents will, as a rule, run in eddies; but if we connect two points in the mass by a conducting wire, these two points being so chosen as to be at different potentials, we thereby modify a portion, at least, of the induced eddy currents' into a current running round a definite circuit.

Experiments.—(i.) If a copper cube be caused to rotate between the poles of a powerful electro-magnet while this latter is as yet un-made,' and if we then 'make' the electro-magnet by sending a current round it, the cube will be visibly retarded or stopped in its movement.

(ii.) A disc caused to rotate in the field will get perceptibly heated. (iii.) In § 3 we shall see how a current may be 'collected' from the disc. (iv.) When a person cuts through the field between the poles of a very powerful electro-magnet with a copper knife, it will appear as though he were cutting through soft cheese, so strong is the opposition due to the induced

currents.

§ 2. Arago's Disc and Magnetic Needle.-There is one case of the above that is of especial historic interest, and though in no way peculiar, will be described at some length. In 1824 Arago discovered the damping' effect produced by the presence of copper and of other conducting masses on magnetic needles oscillating near them. If a needle oscillate very close over a copper disc, and still more if it oscillate between two copper discs,

it will very soon come to rest. This is due to the induction of currents in the copper, these currents being such as to oppose that motion of the needle which is the origin of the induction.

In the figure we have a copper disc caused to rotate with great velocity under a magnetic needle; a sheet of glass between the two obviates any disturbance due to air-eddies caused by the rotating disc. The needle is deflected in the direction of rotation of the disc, and will, if the velocity be great enough, finally rotate also. This motion of the needle is not difficult to explain. In consequence of the rotation of the disc in the magnetic field due to the needle, currents are induced in the former. These currents are in such a direction that they oppose the relative motion of disc

and needle. They would be induced equally if the disc were stationary and the needle rotated. We have thus a reaction between needle and disc that tends to stop the relative motion; and so, if the needle be free to move, it will be urged round in the same direction as the disc. If slits be cut radially these interfere with the induced currents, and therefore with the actions described; but if they be cut in circles whose centres lie on the common axis of rotation of disc and of needle, their presence makes much less difference.

§ 3. Continuous Current Collected from Barlow's Wheel.In the figure the dots represent a field of force, supposed to be running down into the plane of the diagram. The circle is a copper disc, revolving in the direction of the arrow. OBCAO

is a circuit, of which OCA is a wire connected at O with the axis of the disc, and having sliding contact with the edge of the disc at A; this circuit is completed by whatever radius of the disc happens to lie between O and A. If a current from an external source be sent in the direction A OBCA, the disc will rotate in the direction of the arrow. We can consider the field as acting always on the moveable radius O A, urging it (by the law given in Chapter XIX. § 7, and elsewhere) to the left. When so used, the disc is called 'Barlow's wheel. If there be no external source of current, but the disc be forcibly turned with the arrow, there will be a current induced in the circuit in such a direction as to oppose

the motion, i.e. in the direction OAC BO; the seat of the induced E.M.F. being in the shifting radius O A, which is always cutting the lines of force. The current thus induced gives lines of force rising perpendicularly upward from the plane of the diagram, opposed to those of the inducing field.

With a powerful electro-magnet this experiment is very easily performed.

If the rotation be performed in the opposite direction, a very remarkable result follows. The current is of course, in any case, in opposite directions in the portions OA and B C, which we may for simplicity suppose to be parallel. Hence the motion of the disc (when this turns in the contrary direction to that of the arrow), tending as it does always to move O A towards B C, will be against the electro-dynamic repulsion of the parallel and opposed currents in OA and B C respectively. This will give rise to induction opposing the motion; ie. the opposed currents in O A

and B C, or the current of the circuit, will increase in strength. Thus, if a current be started in the circuit, and if then the external field be caused to vanish, induction will continue, owing to the action between the portions OA and BC. We thus have the phenomenon of a current maintained solely by work done on a system of copper conductors; there being no external magnetic field, no battery, and no electricity due to friction.'

If the disc turn as in the figure, the inductive action between BC and O A tends again to oppose the motion; and in this case the effect will be to lessen the current in the portions OA and B C, i.e. the current of the circuit.

§ 4. Induction in the Earth's Field.-We can obtain induced currents by rotation of a coil in the earth's field. In general, when a coil so rotates the number of lines of force piercing it is varied, and there will be E. M.F.s induced.

In the figure, S R is a coil of many turns of insulated wire, and X Y represents the direction of the earth's lines of force. The coil is turned by means of a handle M. The whole is mounted upon a stand in such a way that the axis of rotation may lie in any direction whatever. If the coil be initially in the position shown, viz. perpendicular to the lines of force, then as it is turned

The apparatus as so used is called Sir W. Thomson's electric current accumulator,'