64 



NA TURE 



\_May 23, 1872 



AN EXPERIMENT TO ILLUSTRATE THE 

 INDUCTION ON ITSELF OF AN ELECTRIC 

 CURRENT 



IT is well known that the Eiidden development of .t 

 current in a conductor is opposed by an influence 

 analogous to the ///tv/A? of ordinary matter. A powerful 

 movement of electricity cannot be suddenly produced ; 

 neither can it be suddenly stopped. One consequence is 

 that a periodic interruption of a circuit in which a con- 

 stant electromotive force acts is suflicient, when the self- 

 induction is great, to stop all sensible current, even al- 

 though the interruptions themselves may be of very short 

 duration. Before any copious How can be produced the 

 circuit is broken, and the work has to be begun over 

 again. Whether in any particular case the influence of 

 self-induction is paramount, or not, will depend also on the 

 resistance of the circuit, and on the rapidity of the in- 

 ttrmittencc. The magnitudes which really come into 

 direct comparison are the interval between the breaks, 

 and the time which would elapse while a current geneia- 

 ted in the circuit, and then left to itself, falls to a specific 

 fraction (such as one-half) of its original magnitude. In 

 ordinary cases the duration of transient currents is but 

 a small part of a second of time, so that, in order to 

 bring out the effects of self-induction, the breaks must 

 recur with considerable rapidity. 



There is, however, one remarkable exception to the 

 general lulc, which occurs when, alongside of the principal 

 coil to which the sluggishness is due, there exists an in- 

 dependent course along which the electricity can cir- 

 culate. For instance, suppose that a coil with two wires, 

 such as is often used for electro-magnets, is so arranged 

 that one wire is included in the principal circuit, while the 

 ends of the others are joined. The effect of the second 

 circuit is then to neutralise the self-induction of the first, 

 and so to increase largely the current that passes through 

 it. Let us trace the progress of the phenomenon ; sup- 

 posing that the first circuit has been closed for a sufficient 

 time to allow of the development of the full current which 

 can be excited by the actual electromotive force. 



The moment the rupture is complete, the current in the 

 first wire must stop, but another of the same magnitude 

 and direction is at once developed in the neighbouring 

 circuit. In fact, in virtue of its incrlia, the electiical 

 motion tends to continue with as little change as possible, 

 a result which is attained in great degree by the formation 

 of the second current to fill the place of the first. In a 

 short time the induced current would d'minish and 

 become insensible under the operation of resistance 

 (analogous to ordinary friction); but we are supposing 

 that before this lakes place to any considerable extent 

 the contact is renewed, and the electromotive force again 

 begins, in the first circuit, to push the electricity on. It 

 is now that the peculiarity of the arrangement manifests 

 itself. The current instantly transfers itself back again 

 to the first circuit, which thus, without any delay, has the 

 advantage of the full current which the electromotive 

 force can sustain. If it had not been for the second 

 circuit and its current, the development in the first would 

 only have been gradual, and by supposition so slow that 

 it would be checked by another interruption before any 

 considerable progress could be made. In shoit, the self- 

 induction of the principal circuit is virtually destroyed.* 

 In my experiment the principal circuit consisted of a 

 Sniee cell and one wire of a coil belonging to a large 

 electro-magnet, and which I may call A. The interrupter 

 was a tuning-fork, arranged after Ilelmhollz, and set into 

 regular vibrations of about 128 per second by an inde- 

 pendent current and battery. The fork itself was forged 



* Mathematicians familiar with the theory of electricity will follow this by 

 putting the three induction co-efficients (in Maxwell's notation, L, M, N) 

 equal, and the resistance 0^ the second circuit, S, equal to zero. 



by the village blacksmith, and the whole affair was 

 home-made. Across one prong was placed a sort 

 of rider of copper wire, dipping on either side into 

 a mercury cup, and so arranged that during the 

 vibration its ends should enter and leave the mer- 

 cury, thereby establishing and interrupting the con- 

 tinuity of the circuit. The current was measured by 

 means of a short wire galvanometer whose electrodes were 

 connected with two neighbouring points of the circuit in 

 such a manner that a small but constant proportion of the 

 entire current passed through the instrument. The second 

 wire of the coil. Ao, which is similar to the first and fiut on 

 iviih it, formed the second circuit, when its ends were 

 joined by a short wire. In order to increase at pleasure 

 the effects of induction, iron wires or rods of about a 

 quarter of an inch in diameter were provided, whose in- 

 sertion in the coil materially increased the decisiveness of 

 the result. 



In the first place, the deflection produced on the gal- 

 vanometer when the circuit was permanently completed 

 w-as 58", which fell to 39° when the interrupter was at 

 work, the circuit of A^. being open, and without iron. On 

 closing A, the deflection rose to 46°. A, was again 

 Oldened, and one iron wire introduced, which gave 30°. 

 Two wires'gave 25", while the introduction of thirty re- 

 duced the deflection to 12°. Again closing A,, the read- 

 ing was 43°, raised to 44° only by the removal of the iron. 

 It was clear that the second circuit almost secured the 

 first from the influence of induction, which otherwise 

 greatly reduced the electrical circulation. I may add 

 that the arrangement was very efficient, the galvanometer 

 needle remaining perfectly steady, so that the readings 

 could be taken with ease and accuracy. 



Another experiment made at the same time (about two 

 years ago) may be noticed, if only for its contrast with 

 the preceding. The coil A, being removed from the main 

 circuit, was included in the branch with the galvanometer, 

 as shown in the figure. Here neither the insertion of 



-^QmuM^^A.5)N 



Eivrrnnv 



iron nor the closing of A, made any difference ; the circuit 

 containing the coil remaining always closed, whatever 

 might be the condition of the other. In such circum- 

 stances the average current indicated by the galvanometer 

 is independent of the self-induction of the coil, varying 

 only with the resistance in the branch, and with the 

 a~'i:rage difference of potential at the points of derivation. 



J. W. STRL'TT 



SOME REMARKS ON THE HABITS OF SOME 

 CEYLON AMMALS, AND NOTES ON 

 METHODS FOR KEEPING THEM ALIVE 

 IN CONFINEMENT 



A FTER my duties as member of the Eclipse Expedi- 

 -^~*- tion were over, I spent some time in Ceylon collect- 

 ing natural history specimens for the Oxford .Museum. 

 Besides preserving a large series of animals in solutions, 

 I obtained through the kindness of my friend, Mr. G. H. 

 K. Thwaites, F.R.S,, of Peradeniya, whose kind hospitality 

 I enjoyed, and to whom I am indebted for nearly all my 

 best specimens and information concerning them, various 

 living examples of the Ceylon fauna, and I kept them 

 with more or less success in confinement. Some notes 



