288 Mr. Faraday's Experimental Researches in Electricity. 

 ^ iii. Electrometer and inductive apparatus employed. 



1179. Leaving for a time the further consideration of the 

 preceding facts until they can be collated with other results 

 bearing directly on the great question of the nature of induc- 

 tion, I will now describe the appai'atus I have had occasion 

 to use; and in proportion to the importance of the principles 

 sought to be established is the necessity of doing this so clearly 

 as to leave no doubt of the results behind. 



11 80. Electrometer. The measuring instrument I have em- 

 ployed has been the torsion balance electrometer of Coulomb, 

 constructed, generally, according to his instructions*, but with 

 certain, variations and additions, which I will briefly describe. 

 The lower part was a glass cylinder eight inches in height 

 and eight inches in diameter ; the tube for the torsion thread 

 was seventeen inches in length. The torsion thread itself was 

 not of metal, but glass, according to the excellent suggestion 

 of the late Dr. Ritchief. It was twenty inches in length, and 

 of such tenuity that when the shell lac lever and attached ball, 

 &c. were connected with it, they made about ten vibrations in 

 a minute. It would bear torsion through four revolutions, or 

 1440°, and yet, when released, return accurately to its po- 

 sition ; probably it would have borne considerably more than 

 this without injury. The repelled ball was of pith, gilt, and 

 was 0*3 of an inch in diameter. The horizontal stem or lever 

 supporting it was of shell lac, according to Coulomb's direc- 

 tion, the arm carrying the ball being 2*4 inches long, and the 

 other only 1*2 inches: to this was attached the vane, also de- 

 scribed by Coulomb, which I found to answer admirably its 

 purpose of quickly destroying vibrations. That the inductive 

 action within the electrometer might be uniform in all posi- 

 tions of the repelled ball and in all states of the apparatus, 

 two bands of tin foil, about an inch wide each, were attached 

 to the inner surface of the glass cylinder, going entirely round 

 it, at the distance of 0*4 of an inch from each other, and at 

 such a height that the intermediate clear surface was in the 

 same horizontal plane with the lever and ball. These bands 

 were connected with each other and with the earth, and, being 

 perfect conductors, always exerted a uniform influence on the 

 electrified balls within, which the glass surface, from its ir- 

 regularity of condition at different times, I found, did not. 

 For the purpose of keeping the air within the electrometer in 

 a constant state as to dryness, a glass dish, of such size as to 

 enter easily within the cylinder, had a layer of fused potash 



* Memoires de r Academic, 1785, p. 670. 

 t Phil. Trans., 1830. 



