442 THE POPULAR SCIENCE MONTHLY 



ment upon his own " Doubler," an invention which he had described in 

 Vol. 77 of the Philosophical Transactions. Previous to this Volta had 

 increased the sensitiveness of an electroscope by mounting condensing 

 plates upon it, and Cavallo had still further increased its sensitiveness 

 by using a double condenser. Bennett's first doubler consisted of three 

 brass plates, one of which was mounted upon the standard which sup- 

 ported the gold leaves of his electroscope. The others were provided 

 with insulating handles and were varnished on one side. "When the 

 electroscope had been given a small charge, one plate was laid with its 

 varnished side upon the electroscope plate and touched with the finger. 

 Tt thus received by induction a charge opposite to the charge of the elec- 

 troscope. It was then raised from the electroscope by its insulating 

 handle, and the other plate was laid with its varnished side upon it and 

 touched with the finger. It accordingly received by induction a charge 

 like the charge of the electroscope. It was then touched by its edge to 

 the electroscope plate with which it divided its charge. Both unmounted 

 plates were then discharged, and the process was repeated. By sufficient 

 repetition the charge of the electroscope could be built up to any desired 

 intensity. 



Bennett's doubler was improved by Dr. Erasmus Darwin by mount- 

 ing the plates upon horizontal arms which could be swung into and out 

 of position readily, and later by mounting the plates vertically and 

 moving them back and forth by a rack work in a direction always par- 

 allel to each other. In this form it was used by Darwin in the study of 

 atmospheric electricity. 



Bennett had noticed that his plates nearly always had a residual 

 charge of electricity which made it possible to build up a charge on his 

 electroscope without giving it a preliminary charge. To get rid of this 

 he improved Darwin's form of the doubler by leaving the plates unvar- 

 nished and depending upon the air for insulation. He found that this 

 made it possible for him to thoroughly discharge the apparatus, so that 

 no charge could be built up until a preliminary charge had been given 

 to the electroscope. 



Soon after this Nicholson built a doubler in which two of the plates 

 were fixed and the movable plate was mounted on an arm and turned 

 about an axis by a crank, by which the proper contacts were also auto- 

 matically made. This doubler Nicholson presented to Bennett, and it 

 was used by him in his investigation of the " adhesive " effects of elec- 

 tricity. Bennett's original figure of this doubler is here reproduced. In 

 this figure A is the fixed plate which remains permanently insulated and 

 upon which it was desired to build up the charge. B is the movable 

 plate which is carried on an arm from an axis which may be turned by 

 a crank, and C is a fixed plate which may be either insulated, joined to 

 A or joined to earth. I is a ball which serves to counterpoise the crank 



