96 ELEMENTS OF STATIC ELECTRICITY. 



electricity through any external source ; and its own 

 electricity, being merely changed to the upper or lower 

 surface, by the positive or negative of the dielectric, is 

 restored to zero when removed from that influence. 



This connection may be made automatically f by plac- 

 ing a short brass pin in a hole made through the dielec- 

 tric, its upper end even with the upper surface, so that it 

 shall touch the cover and also the lower plate. This 

 makes the instrument more convenient for obtaining 

 charges in rapid succession : but, when used to demon- 

 strate the principles involved in its construction, as 

 above, the pin should be removed. 



The top of the handle should be grasped, when 

 removing the cover, to prevent a partial discharge 

 through the hand. 



The electrophorus will retain its charge for months ; 

 and, like the Leyden jar with movable coatings, can be 

 taken apart and put together again without perceptible 

 loss of charge ; but, when not in use, the charge is 

 gradually dissipated, so that only a residual remains. 

 Hence it should be charged again before immediate 

 use, if great efficiency is desired. This property of 

 constancy probably suggests the name, electrophorus, 

 electricity-bearer, from qisaco to bear, qfaxTQov electricity. 



THE FKLCTIONAL MACHINE. The principle of this 

 machine is the same as that of the rod and rubber. It 

 was invented by Otto Guericke, and consisted, at first, 

 of a globe of sulphur, revolved on an axis by a crank, 

 the hand being used as a rubber. Subsequently a globe 

 of glass was substituted for the sulphur ; but as insu- 

 lation was disregarded in both styles, only feeble results 

 were obtained, and the machines fell into disuse. 



Boze, of Wittemberg, revived and improved them, 



