CoulomU's Electrometer — particular Adjustments, 289 



placed within it, and this being covered with a disc of fine 

 wire gauze to render its inductive action uniform at all pans, 

 was placed within the instrument at the bottom and left 

 there. 



1181. The moveable ball used to take and measure the por- 

 tion 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 accord- 

 ing 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. 



1 182. 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 conductor 

 that it neither receives nor gives a charge freely, and so, after 

 contact with a charged conductor, is liable to be in an uncer- 

 tain condition. Again, it is difficult to turn pith so smoothly 

 as to leave the ball, even when gilt, sufficiently free from ir- 

 regularities of form, as to retain its charge undiminished for 

 a considerable length of time. When, therefore, the balls 

 are finally prepared and gilt they should be examined, and 

 being electrified, unless they can hold their charge with very 

 little diminution for a considerable time, and yet be discharged 

 instantly and perfectly by the touch of an uninsulated con- 

 ductor, they should be dismissed. 



1183. It is, perhaps, unnecessary to refer to the graduation 

 of the instrument, further than to explain how the observa- 

 tions were made. On a circle or ring of paper on the outside 

 of the glass cylinder, fixed so as to cover the internal lower 

 ring of tin foil, were marked four points corresponding to an- 

 gles of 90° ; four other points exactly corresponding to these 

 points being marked on the upper ring of tin foil within. By 

 these and the adjusting screws on which the whole instrument 

 stands, the glass torsion thread could be brought accurately 

 into the centre of the instrument and of the graduations on 

 it. From one of the four points on the exterior of the cylin- 

 der a graduation of 90° was set off", and a corresponding gra- 

 duation was placed upon the upper tin foil on the opposite 

 side of the cylinder within ; and a dot being marked on that 

 point of the surface of the repelled ball nearest to the side of 



Phil, Mag. S. 3. Vol. 13. No. 82. Oct, 1838. U 



