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VI. On a permanent Deflection of the Galvanometer-needle 

 under the influence of a rapid series of equal and opposite in- 

 duced Currents. By Lord Rayleigh, F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



GrENTLEMEN, 



THE publication, in your December Number, of a memoir 

 by Mr. Chrystal on Bi- and Unilateral Galvanometer 

 Deflection recalled to my mind some observations of a like 

 character made some years ago by myself. I have lately suc- 

 ceeded in finding the manuscript of a communication with 

 the above title read (literally) before the British Association 

 at Norwich in 1868, which contains a short account of these 

 observations. As the subject has acquired an additional inter- 

 est in consequence of the investigations of Dr. Schuster and 

 Mr. Chrystal, I shall be glad if you can find room for my 

 paper, which has not been printed in full hitherto. 

 I am, Gentlemen, 



Your obedient Servant, 

 Terling Place, Witham, RAYLEIGH, 



December 11, 1876. 



The following paper contains a short account of some ex- 

 periments which led to rather unexpected results, of which 

 I can find no notice in the methodical treatises on Electricity, 

 although they might seem to be in the way of any experi- 

 menter on induced currents. The arrangement of the first 

 experiment was nearly the same as that described by Faraday 

 in his original memoir on induction. Two thick copper wires 

 were coiled together — the circuit of one being completed by 

 the battery and make-and-break apparatus, and that of the 

 other by an ordinary astatic galvanometer of moderate sensi- 

 tiveness. The make-and-break arrangement was a very rude 

 one of my own construction, acting either by the dipping of 

 needles into mercury, or by the intermittent contact of a spring 

 with a toothed wheel. When the handle of the instrument is 

 turned, there are generated in the second circuit, as is well 

 known, a series of instantaneous currents which are alternately 

 opposite in sign but whose magnitudes are equal, although 

 that corresponding to the break of the battery-circuit is the most 

 condensed. When, then, the instrument is worked with such 

 rapidity that the interval between the currents is very small 

 in comparison with the time of free oscillation of the needle, 

 the latter might be expected to be sensibly unaffected. But 

 so far was this from being the case, that although the swing of 



