16 



STATIC ELECTRICITY 



troublesome and slow to gather together any large quantity in this 



way. 



3A 



Bennett's Doubler. Very soon after the invention of the 

 electrophorus means were devised for increasing the output, the 

 earliest being Bennett's Doubler, invented in 1786 for the purpose 

 of increasing very weak charges. It consisted of three metal 

 plates A B C, Fig. 13, varnished as indicated by the black lines, 

 A on the upper side, B on both sides, and C on the under .side, 

 the varnish acting as an insulator. B and C were provided with 

 insulating handles. Let us suppose the following operation to be 



gone through : A is placed on an 

 insulator, which we may suppose to 

 be tin electroscope as that will ser\e 

 as an indicator of increasing charge ; 

 B is placed on A. If a conducting 

 body charged, say, positively touches 

 A, while B is touched to earth, 

 almost all the charge will galli- 

 A, for there it can be quite do>e to 

 the opposite negative charge induced 

 on the under side of B. This nega- 

 tive charge is nearly equal to that 

 on A. Now insulate B and remove 

 the body; let B IK- raided and 

 brought again>t C; touch C to 

 earth. A positive charge nearly 

 equal to the negative on B or the 

 positive on A gathers on ( '. In>ulute 

 C and replace B on A; touch B to earth; touch (' to A, uhen 

 nearly the whole of its charge will pass to A. being added to and 

 thus almost doubling the original charge. Repeating the process 

 after each series of operations, the charge on A is nearlv doubled 

 and may soon be made a very large multiple of its original value, 

 as the electroscope will show. The disadvantage of this form of 

 apparatus is that it requires considerable manipulation, and it \\a> 

 soon followed by instruments in which the process was carried out 

 mechanically by the turning of a handle. Probably they were not 

 very efficient, for they seem to have been considered merely as 

 curiosities. One machine, invented by Belli in 1831, should have 

 secured more notice. It appears, however, to have been neglected, 

 and it was not until the independent invention of machines bv 

 Varley, Thomson, Toepler, and Holt/, about the years 1860-65, that 

 general attention was directed to the subject. Since that time 

 various forms of machines, all dependent on induction or " in- 

 fluence "and not on friction, have been devised. These are termed 

 Influence Machines, and have now practically displaced the old 

 frictional machines as sources of electrification. As, perhaps, the 

 simplest in principle we shall first describe Belli's machine. 



FIG. 13. 



