1 84 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



true only for plates which are very large in comparison with their 

 distance apart, and this guard ring makes it possible to realize 

 this condition approximately without making the moving part a 

 of the upper plate inconveniently large. 



The value of the constant B t which appears in equation (62), 

 Art. 90, which also appears in the equations for electrostatic 

 attraction, and which is equal to the amount of electric flux which 

 emanates from one coulomb according to equation (69), may be 

 most easily determined by means of the absolute electrometer. 

 A known electromotive force E is connected to the plates of an 



& 



lc 'fate 

 Suspending 

 wire 



Sectional view 



(op view 

 Fig. 1 1 3a. 



Fig. 1 1 3b. 



absolute electrometer, and the force F is measured, the area a 

 of the movable plate and the distance x between the plates be- 

 ing known, then the value of B may be calculated from equa- 

 tion (74), k being equal to unity for air. 



103. The electrostatic voltmeter consists essentially of a fixed 

 metal plate and a delicately poised or suspended metal plate 

 which carries a pointer which plays over a divided scale. The 



