282 



ELECTROSTATICS. 



[PT. II. CH. VI. 



approach too near, the force becomes greater and the plate is 

 ^ -^ attracted still nearer, and is accordingly in un- 



stable equilibrium. 



The electrometer of Bichat and Blondlot* 

 consists of a cylinder with rounded ends inserted 

 concentrically into a hollow cylinder and sup- 

 ported by a balance. If the ends of the internal 

 cylinder are far enough from the end of the 

 external cylinder, the distributions upon them 

 will be independent of the depth to which the 

 internal cylinder enters the other. For a certain 

 distance the field between the cylinders (whose 

 equipotential surfaces are shown in Fig. 59) 

 will be the same as if the cylinders were 



of infinite length. Let the length of this portion be I. Then 



we have 



\ 



FIG. 59. 



and the force tending to increase Z, that is, to draw the inner 

 cylinder into the outer, is 





The difference of potential is proportional to the square root of 

 the force, and independent of the position of the inner cylinder, 

 provided only that the cylinders be long enough. This electro- 

 meter is of course less entitled to the designation absolute than 

 Lord Kelvin's, on account of the assumptions made. 



We have now seen that in any absolute measurement of 

 potential, we must measure a force and certain geometrical 

 quantities of the nature of lengths. 



* Journal de Physique, 2 me . S6rie, t. v. 



