362 J. Trowbridge— Thin plates of iron used a 



various sizes were placed in the primary circuit. The results 

 given in this paper were obtained by the use of a condenser of 

 about one farad. The method of experimenting was to charge 

 a condenser of one-third of a farad ; and then to discharge this 

 condenser through a galvanometer. If we express the quantity 

 of electricity received by the condenser by Q, the electromo- 

 tive force by E, and the capacity of the condenser by C, we 

 have Q=— sin i ^, where n is the reduction factor of the 

 galvanometer, t the time of vibration of the magnet, and 6 the 

 angle through which it swings under the effect of the change. 

 Knowing the reduction factor of my galvanometer, I had thus 

 the means of reducing my results to absolute measure. But I 

 speedily found that the relative results obtained by the pro- 

 portions 



Q: Q'=sinifl; sin ^ ^'=E : E' 

 would present the points of this investigation in as clear a man- 

 ner as if the results had been reduced to absolute magnetic 

 measure. My first experiments were made with solid arma- 



60 70 I 60 400 



80 90 I 80 400 



Mean, 75 86 I Mean, 75 Mean, 386-6 



In table I the numbers are the deflections of the reflecting 

 galvanometer expressed in millimeters, and the distance of the 

 scale from the magnet was one meter. In this case the gain by 

 the use of the armatures was trifling, being only about four- 

 teen per cent. These results were obtained by charging the 

 condenser of one-third of a farad, by sparks one millimeter in 

 length. On a closed secondary circuit, however, a gain of one 

 hundred per cent was clearly seen in the strength of the in- 

 duced currents produced by breaking the primary circuit. The 

 question, how to make this increase in the strength of the in- 

 duced current by the employment of armatures apparent on a 

 broken secondary circuit, became an interesting one. It seemed 

 at first as if the application of armatures, by maintaining the 

 temporary magnetization of the iron cores would be detri- 

 mental rather than otherwise. I next tried the effect of bun- 

 dles of thin iron plates, which were placed, as armatures, upon 



