tLECTIlOM AGNETI C IN D U CTiOX. ] ELECT It I C I T Y duce.l others, and so cm. 1 This can be brought about by forming rrents part of the secondary circuit of one inductive apparatus higher ^ ^ Q p r j mar y O f the next, and so on. As may be ers> supposed, the successive induced currents diminish very rapidly in strength, and require special means for their detection. But the phenomenon also goes on increas ing in complicacy. Suppose we start the current in the first primary, there is a single inverse current of the "first order" which rises and then falls; there will, there fore, be two currents of the " second order " first a direct, then an inverse ; each of these rising and falling causes two currents of the third order, and so on in geometric- progression. These currents have been detected in certain cases by means of their physiological action and their mag netizing powers. The latter effects present some points of interest in connection with magnetism, but we cannot spare space for the matter here. Self-induction. The existence of self-induction has been deduced as a theoretical consequence of the general law of induction. It was not so discovered, however. It was first arrived at by Faraday 2 from experimental considera tion s tions. The observation from which he started was the ?erva- following fact communicated to him by Mr Jenkin, who had ns- shortly before discovered it : Although it is impossible with a short circuit of wire and a single battery cell to obtain a shock by making and breaking contact, yet a very powerful shock is obtained if the coil of an electromagnet be included in the circuit. This may be shown thus : Let ZC (fig. 50) be a battery of a single cell, CABDEF a circuit with a cross branch BF, in which at G the human body, &c., may be in serted. Contacts can be made and broken at A, very rapidly if need be, by means of a toothed wheel. When BDEF consists of a short single wire, nothing particular is felt at G, but when the coil of an electromagnet is inserted in DE, the patient at G experiences a series of powerful shocks comparable to that obtained from the secondary coil of an inductive apparatus in the manner already described. ra- If the cross circuit be done away with, a powerful spark is obtained at A on breaking contact, but none on making. This spark is particularly bright if a mercury contact be used, owing to the combustion of the mercury. If, how ever, the electromagnet be removed from DE, and a short wire substituted, the spark becomes quite insignificant, al though the whole circuit may be now very hot, owing to the increased current. Faraday found that the same effect, ouly smaller, was produced when a simple helix without a core was substituted for the electromagnet ; and a similar effect, only still smaller, was obtained when a very long straight wire was used. Faraday soon recognized that these effects are consequences of the laws at which he had arrived in his first series of researches on induction. When the current is rising in a circuit, the number of lines of magnetic force passing through it is on the increase, hence an electromotive force is generated which opposes that of the battery, and causes ths current to rise slowly; again, when the current begins to decrease the number of lines of force begins to decrease, and an electromotive force of induction is called forth which tends to prolong the current. We have, therefore, a weakening of the electromotive force at starting and an exaltation at stop ping, which accounts for the absence of the spark or shock at make, and the presence of one or other at break. Such 1 Some physicists have called these currents induced currents of the second and third orders, &c. 2 E.CI>. lies., 1048, &c., 1834. Fig. 50. inductive effects are obviously heightened when the cur rent is wound into a spiral form; if, however, the spiral were wound double, and the current sent through the two wires in opposite directions, the inductive effects would annul each other, and, in fact, with this arrangement the spark and shock are extremely small. Faraday demonstrated the existence of these electronic- Hi s er- tive forces by means of the currents which they produce peri- in derived circuits, 3 when the battery contact is broken ments - or made. He used the arrangement given in figure 50. A galvanometer was inserted at G, and the needle stopped by pins properly placed from deviating as urged by the branch of the battery current from B to F, but left free to move in the opposite direction. It was found that the needle deviated sharply when contact was broken at A, in a direction indicating a current from F to B. Again the con tact was made, and the needle stopped at the deviation due to the current from B to F, so that it could not return to zero. The contact was then broken and made again, and it was found that at the make the needle tended to go beyond the position due to the steady current in BF. Faraday also arranged a platinum wire at G, so that it did not glow under the steady current in BF, but immediately ignited when the contact at A was broken. Chemical action was produced in a similar manner. In fact we may, by taking advantage of the self-induction, cause a single cell to produce decomposition of water and evolution of gas, which it could not do alone consistently with the conservation of energy. This may be managed 4 by inserting at A (fig. 50), instead of the contact breaker, the coil of an electro magnet, and placing the decomposing cell in DE. Let contact be made aud broken at G (say by an automatic break); when the con tact is made the current flows through the coil and through BF, when it is broken the electromotive force of induction added to that of the battery enables the current to pass through the cell and liberate the ions. At the make there is no such effect; there results therefore continued chemical decomposition. Edlund 5 investigated the integral electromotive forces of Edluud s self-induction at the opening and closing of a circuit, and measure- showed that they are equal. His experimental arrangement ments of is very ingenious : G (fig. 51) is a differential galvanometer, A a coil whose self-in duction is to be examined, C a wire wound in a zig-zag 6 so as to have no self-induction. The battery E is connected at B and D with the circuit composed of G, A, and C, so that the currents in BccZCD and B&fflAD pass round the coils of G in opposite directions. The resistance C is so arranged that there is no deflec tion of the needle in G. K now the current be stopped by breaking the circuit EB atK, the electromotive force . due to the self-induction of A causes an extra current to flow round the circuit AabHcdCD, traversing the coils of G in the same direction. "We there fore get a deflection D l . In a similar manner if we make contact at K we get another deflection D 2 , due to the starting of the current in A. There is no difficulty in showing that, if E t , E 2 be the time integrals of the electro motive force in the two cases, then EI D, E ^IV One of the difficulties encountered in such experiments is the increase of the electromotive force of the battery E when it is left open for a time ; this causes the extra cur rent at make to be greater than that at break. Rijke, who made experiments similar to those of Edlund, avoids this difficulty by circuiting the battery, when BK is broken, B Fig. 51. 3 These currents are sometimes called extra currents, and the nanc is applied even when there is no alternative circuit. The extra cur rents are then the defect or excess of the currents at the mako and break, considered with reference to the steady current. 4 De la Rive, Wiedemann, Bd. ii. 740. 9 Pogft. Ann., 1819. 6 The best, arrangement would be to use insulated wire and double
it on itself.