336 Mr. M. Donovan on the supposed Identity of the Agent 



" charged by thirty turns of the machine, and discharged through 

 the galvanometer, a thick wet string about ten inches long being 

 included in the circuit. The needle was immediately deflected 

 22°*." 



Seven more jars were then added to the eight, and the whole 

 fifteen were charged by thirty turns of the machine. When the 

 discharge was passed through the galvanometer, the needle de- 

 viated exactly to the 22nd degree as before. The whole battery 

 of fifteen jars was now charged with fifty turns of the plate, and 

 the discharge was made through the galvanometer by means of 

 various media. Through a thick wet string, the charge passed 

 at once ; with a thin string, it occupied a sensible time ; and 

 with a thread, it required two or three seconds. " The current, 

 therefore, must have varied extremely in intensity in these dif- 

 ferent cases, and yet the deflection of the needle was the same in 

 all of them f." Hence he infers the law already mentioned, that 

 the same quantity of electricity affects the magnetic needle 

 equally, no matter what the intensity. 



In the first place, it is to be observed that this law has not 

 received universal assent. Pouillet lays down the very reverse 

 of it : he says, " We may call currents of the same intensity those 

 which produce the same deflection :" "a current will have double 

 or triple the intensity of another current when it produces de- 

 flections of which the sines are double or triple," as indicated by 

 the compass of sines J. 



The experiments of Faraday, and the inference drawn from 

 them, are very important, and require some scrutiny. He found, 

 it is true, that thirty turns of the plate produced a quantity of 

 electricity, which, whether received into eight or fifteen jars, and 

 passed through the galvanometer, occasioned the same degree of 

 deflection. But in this there appears nothing but what might 

 have been expected and ought to happen. The quantity of elec- 

 tricity was the same in both cases : the intensity certainly dif- 

 fered when eight or fifteen jars were used, that is, while the 

 electricity was contained in the jars ; but in the act of dischar- 

 ging them through the galvanometer, the whole quantity, whether 

 from eight or fifteen jars, passed through the coil, and was raised 

 at that instant to the intensity which the small surface of the 

 wire of the coil condensed it into and determined. It is quite 

 indifferent whether the electricity of thirty turns of the plate- 

 machine were diffused over the coated surface of eight jars, or 

 fifteen, or one hundred ; for although the intensity of that quan- 

 tity while in the jars would, according to their number, be very 

 different, yet the whole charge must pass from all the jars at the 



* Researches, par. 363. f Ibid. 3(55. 



% Elements cle Physique, vol. i. p. 32fi. 



