2i 4 ELEMENTARY LESSONS ON [CHAP. iv. 



Sir W. Thomson has introduced several very import- 

 ant modifications into the construction of attracted-disc 

 electrometers, the chief of these being the employment 

 of the " guard-plate " and the providing of means for 

 working with a definite standard of potential. It would 

 be beyond the scope of these lessons to give a complete 

 description of all the various forms of attracted-disc 

 electrometer ; but the main principles of them all can be 

 readily explained. 



The disc, C, whose attraction is to be measured, is sus- 

 pended (Fig. 100) within a fixed guard-plate, B, which 



Fig. 100. 



surrounds it without touching it, and which is placed 

 in metallic contact with it by a fine wire. A lever, L, 

 supports the disc, and is furnished with a counterpoise ; 

 whilst the aluminium wire which serves as a fulcrum may 

 be also employed to produce a torsion force. In order 

 to know whether the disc is precisely level with the 

 lower surface of the guard-plate a little gauge or index 

 is fixed above, and provided with a lens, /, to observe 

 its indications. Beneath the disc and guard -plate is 



