CHAPTER XL 

 ELECTROMETERS. 



PROGRESS in every department of science is largely 

 dependent on exact measurement, since it is only by 

 this means that we get an accurate knowledge of 

 relative values. The thermometer enables us to in- 

 vestigate the laws of heat; the barometer gives us a 

 knowledge of atmospheric pressure, and the various 

 matters relating to it. And in chemistry and astron- 

 omy almost every step depends on such measurement. 

 Even our ordinary business transactions, and the value 

 of our currency, are regulated by the common scales, 

 by which we measure the force of gravity. 



Electric science is no exception to this rule. We 

 require to know, accurately, relative differences of 

 potential; the conductivity and resistance of various 

 substances ; the force of electric attraction and repul- 

 sion, the comparative energy of the various instruments 

 used for generating and accumulating electricity ; and 

 other matters of similar importance. 



But electric measurement presents peculiar difficul- 

 ties not met with in the measurement of other forms 

 of energy. In the measurement of gravity, we deal 

 with a force easily controlled, the direction of whose 

 movement is always known, and which, on the various 

 parts of the earth's surface, is subject to but slight 

 variation. 



