156 



ELEMENTS OF STATIC ELECTRICITY. 



In heat we have a force, susceptible of easy control ; 

 its movement slow, and its direction easily ascertained. 

 But electricity moves with the rapidity of thought; 

 its direction is difficult to ascertain ; and it defies our 

 utmost efforts at absolute control; so that the results 

 of measurement, by our best constructed instruments, 

 fall short of perfect accuracy. 



In static electricity less progress has been made in 

 measurement than in other forms of electric energy, 

 whose practical applications are more numerous. 



The electroscope, sometimes 

 classed with electrometers, in- 

 dicates the presence of an elec- 

 tric charge, but cannot be said 

 to measure it, except as such 

 indication may show an in- 

 crease or diminution of a light 

 charge. Lane's unit jar may 

 be considered an electrometer, 

 and the methods of measure- 

 ment by it, and by sparks 

 from the Holtz and Topler 

 machines, belong to the same 

 class : but both methods are 

 very inaccurate, and can be 

 used only in special cases. 



COULOMB'S TORSION BALANCE. To Coulomb is 

 due the credit of the first efforts at accuracy in elec- 

 tric science ; and the torsion balance, which is still 

 extensively used, was his invention and may properly 

 be regarded as the first electrometer. 



It is represented by Fig. 50; and consists of a glass 

 cylinder A J., to the top of which is attached, at the 



Fig. 50 Coulomb's Torsion 

 Balance. 



