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



sixteenth jar made no difference in the results; the double quantity 

 of electricity was present. The inference drawn was, that the 

 angular deflection was doubled because the quantity of electricity 

 was doubled. The deflection and deflecting force would be in 

 the same ratio, if every degree on the quadrant of the galvano- 

 meter were of equal value with regard to the resistance which 

 terrestrial magnetism offers to the deflection of the needle*. 

 But every experiment proves that this is not the case. It is 

 proved by the researches of Lambert and Coulomb, that the 

 effect of terrestrial magnetism on a freely suspended magnetic 

 needle is as the sine of the angle formed by the magnetic meri- 

 dian with the magnetic axis of the needle. The sine of an angle 

 of 22° being taken as =2, the sine of an angle of 44° would be 

 V854; to have doubled the sine, i. e. the force which produced 

 a deflection of 22°, would require that the needle in the second 

 instance should have stood at 484°. The law of Lambert would 

 apply to every degree in the quadrant of a galvanometer if it 

 had but one wire, and if that one coincided with the magnetic 

 meridian; but in galvanometers with a coil, the law does not 

 apply. Melloni has shown, that when the needles are nearly 

 astatic, the first 20° are in the direct ratio of the deflecting 

 forcesf ; but in the galvanometer of his construction, all degrees 

 above 20° bear a different value. According to Melloni's table, 

 Faraday's results would stand thus : the deflection 22° would 

 indicate a force =22'3, but the deflection 44° would indicate a 

 force =78. Hence when the needle pointed to 44°, instead of 

 indicating the passage of double the quantity of electricity that 

 traversed the wire when the deflection was 22°, it represented it 

 just three and a half times greater. Thus it would appear that 

 Faraday's experiments do not support the law that " the deflect- 

 ing force of an electric current is directly proportional to the 

 absolute quantity of electricitypassed," but are at variance with it. 

 If the law itself fail, the comparison which he has drawn be- 

 tween the quantity of electricity produced during the chemical 

 action of acidulated water on certain wires (to be immediately 

 noticed), and that discharged from an electric machine, cannot 

 be considered as proved. For it may be true that quantity has 

 no more to do in the phenomenon than in the indications of Hen- 

 ley's electrometer ; that intensity is the real condition for causing 



* Pouillet, in describing a galvanometer with a coil, says " the deflec- 

 tion increases with the intensity of the current ; but we know that it cannot 

 in any manner be proportional to this intensity." — Elements de Physique, 

 vol. i. p. 501. 



Faraday has not given the ratio of deflection to the deflecting force in 

 his galvanometer. 



t This holds true also according to the law of Lambert, for the sine of 

 20° is double the sine of 10°. 



