320 



foundation of various important estimates in several departments 

 of electrical science. 



The absolute electrometer alluded to above, consists of a plain 

 metallic disc, insulated in a horizontal position, with a somewhat 

 smaller plane metallic disc hung centrally over it, from one end of the 

 beam of a balance. A metal case protects the suspended disc from 

 currents of air, and from irregular electric influences, allowing a light 

 vertical rod, rigidly connected with the disc at its lower end, and sus- 

 pended from the balance above, to move up and down freely, through 

 an aperture just wide enough not to touch it. In the side of the 

 case there is another aperture, through which projects an electrode 

 rigidly connected with the lower insulated disc. The upper disc is 

 kept in metallic communication with the case. 



In using this instrument to reduce the indications of an electro- 

 scopic or torsion electrometer to absolute electrostatic measure, the 

 insulated part of the electrometer is kept in metallic communication 

 with the insulated disc, while the cases enclosing the two instruments 

 are also kept in metallic communication with one another. A charge, 

 either positive or negative, is communicated to the insulated part of 

 the double apparatus. The indication of the tested electrometer is 

 read off, and at the same time the force required to keep the move- 

 able disc at a stated distance from the fixed disc below it, is weighed 

 by the balance. This part of the operation is, as I anticipated, 

 somewhat troublesome, in consequence of the instability of the equi- 

 librium, but with a little care it may be managed with considerable 

 accuracy. The plan which I have hitherto followed, has been to 

 limit the play of the arm of the balance to a very small arc, by 

 means of firm stops suitably placed, thus allowing a range of motion 

 to the upper disc through but a small part of its whole distance from 

 the lower. A certain weight is put into the opposite scale of the 

 balance, and the indications of the second electrometer are observed 

 when the electric force is just sufficient to draw down the upper disc 

 from resting in its upper position, and again when insufficient, to keep 

 it down with the beam pressed on its lower stop. This operation is 

 repeated at different distances, and thus no considerable error de- 

 pending on a want of parallelism between the discs could remain 

 undetected. It may be remarked that the upper disc is carefully ba- 

 lanced by means of small weights attached to it, so as to make it hang 



