ELECTROMETERS. 



TWO LECTURES. 



BY J. T. BOTTOMLEY, M.A., F.R S.E., DEMONSTRATOR OF NATURAL 

 PHILOSOPHY IN THE UNIVERSITY OF GLASGOW. 



LECTURE I. 



WHEN a difference of electric potentials exists between 

 two points it gives rise, or it may give rise, to one of two 

 effects : an electric current may be produced, or there 

 may be an exhibition of electrostatic force. Either of 

 these effects may be employed for measuring differences of 

 electric potential, and hence we have two classes of instru- 

 ments for this purpose. Thus we have galvanometers, 

 electro-dynamometers, voltameters, in which the difference 

 of potentials between two points is inferred from the 

 measurement of the current passing between those points 

 under known circumstances. On the other hand, we have 

 electroscopes and electrometers which indicate and measure 

 differences of electric potential by means of the effects of 

 electrostatic force ; that is by means of the attractions or re- 

 pulsions observed between electrified bodies. An electroscope 

 merely gives an indication of the existence of the difference 

 of potentials ; an electrometer, properly so called, measures 

 the amount of the difference. The first requisite of an 

 electrometer is to furnish numbers, by the scale readings, 

 which are proportional to the difference of potentials 

 between the points tested. From these numbers we must 



