50 ELEMENTARY LESSONS ON [CHAP. i. 



some other point where they can increase the initial 

 charge, or furnish a supply of electricity to a suitable 

 collector. Of such instruments the oldest is the Elec- 

 trophorus of Volta, explained fully in Lesson III. 

 Bennet, Nicholson, Darwin, and others, devised pieces 

 of apparatus for accomplishing by mechanism that which 

 the electrophorus accomplishes by hand. Nicholson's 

 revolving doubler consists of a revolving apparatus, 

 in which an insulated carrier can be brought into the 

 presence of an electrified body, there touched for an 

 instant to remove its repelled electricity, then carried 

 forward with its acquired charge towards another body, 

 to which it imparts its charge, and which in turn acts 

 inductively on it, giving it an opposite charge which 

 it can convey to the first body, thus increasing its 

 initial charge at every rotation. Similar instruments 

 have been contrived by Varley, Sir W. Thomson (the "re- 

 plenisher"), Topler, Carre, and Holtz. The two latter 

 are perfectly continuous in their action, and have been 

 well described as continuous electrophori. The machine 

 of Holtz has come into such general use as to deserve 

 explanation. 



46. Holtz's Electrical Machine. The action of 

 this machine is not altogether easy to grasp, though in 

 reality simple enough when carefully explained. The 

 machine consists (see Fig. 29) of two plates, one, A, 

 fixed by its edges ; the other, B, mounted on an axis, and 

 requiring to be rotated at a high speed by a band and 

 driving pulley. There are two holes or windows, P and 

 P', cut at opposite points of the fixed plate. Two pieces 

 varnished paper, f and f are fastened to the plate above 

 the window on the left and below the one on the right. 

 These pieces of paper or armatures are upon the side 

 of the fixed plate away from the movable disc, or, as 

 we may say, upon the back of the plate. They are 

 provfded with narrow tongues which project forward 

 through the windows towards the movable disc, which 



