74 ELEMENTS OF STATIC ELECTRICITY. 



glass. But since the resistance of glass is in the 

 direct ratio of its thickness, when the specific induct- 

 ive capacity is the same, this factor also must be 

 considered. 



Hence, in the construction of instruments involving 

 these principles, if great sensitiveness and a low poten- 

 tial is desired, the glass, or other dielectric, should be 

 thin : but if the highest attainable potential is desired, 

 there should be sufficient thickness to resist fracture 

 or puncture. 



The uncoated margin must also be wide enough to 

 m^ike the resistance there equal to that of the thickness ; 

 a small fraction of an inch in thickness having a re- 

 sistance equal to that of several inches of surface. 



No definite rules can be given, as the resistance of va- 

 rious kinds of glass, and other dielectrics, varies greatly, 

 as well as the cases in which they may be required. 



As the positive and negative on opposite surfaces are 

 equal, it is impossible for a change of potential to occur 

 on either surface without a corresponding change on 

 the opposite surface. Hence a conductor brought into 

 contact with either surface alone will not change its 

 potential, unless directly or indirectly connected with 

 the opposite surface. Hence the charge on each surface 

 is said to be bound by the opposite charge. 



The convection and conduction of the air, so far 

 as it can act equally on both surfaces, will in time re- 

 store equilibrium. It may also be restored by the oscil- 

 lation of a solid body, as a pith ball, suspended between 

 conductors connected with both surfaces ; or, by direct 

 connection through a conductor, as already explained. 



Instruments constructed for accumulating electricity 

 in this way are called accumulators, or condensers. 



