Page:Popular Science Monthly Volume 83.djvu/446

442 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 supported 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 electroscope. 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 re]3etition the charge of the electroscope could be built up to any desired intensity.

Bennett's doubler was improved by Dr. Erasmus Darwin by mounting 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 parallel 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 unvarnished 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 wore also automatically made. This doubler Nicholson presented to Bennett, and it was used by him in his investigation of the "adhesive" effects of electricity. 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