coulomb's electrometer — PARTICULAR ADJUSTMENTS. ^ 



1180. Electrometer. The measuring instrument I have employed 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 dia- 

 meter ; 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. Ritchie -j~. 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 position ; 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 direction, the arm carrying the ball being 2*4 inches 

 long, and the other only 1*2 inches : to this was attached the vane, also described by 

 Coulomb, which I found to answer admirably its purpose of quickly destroying vibra- 

 tions. That the inductive action within the electrometer might be uniform in all 

 positions 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 irregularity 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 placed within it, and this being covered with 

 a disc of fine wire gauze to render its inductive action uniform at all parts, was placed 

 within the instrument at the bottom and left there. 



1181. The moveable ball used to take and measure the portion of electricity under 

 examination, and which may be called the repelling, or the carrier, ball, was of soft 

 alder wood, well and smoothly gilt. It was attached to a fine shell lac stem, and 

 introduced through a hole into the electrometer according to Coulomb's method: 

 the stem was fixed at its upper end in a block or vice, supported on three short feet; 

 and on the surface of the glass cover above was a plate of lead with stops on it, so 

 that when the carrier ball was adjusted in its right position, with the vice above 

 bearing at the same time against these stops, it was perfectly easy to bring away the 

 carrier ball and restore it to its place again very accurately, without any loss of time. 



1182. It is quite necessary to attend to certain precautions respecting these balls. 

 If of pith alone they are bad ; for when very dry, that substance is so imperfect a 



* Memoires de TAcademie, 1785, p. 570. 

 t Philosophical Transactioiis, 1830. 



