11 



springs; but, though disconnected, each wing retains its charge. 

 Further rotation brings the right-baud wing with the positive 

 charge in contact with the left-hand shield, and the charge is sent 

 to the jar. Tlie negative electricity on the left-hand wing runs to 

 the outer coating of the jar. 



The shields are now neutral, as at first, but by continuing 

 the rotation the process is repeated. Every turn increases the 

 potential of the jar, and we can augment it as much as we choose. 

 By reversing the motion we can likewise diminish the charge. 

 The instrument is so extremely sensitive to slight variations of 

 potential that a few turns of the replenisher will supply any loss, 

 however small. 



Water-dripping Apjmratvs. — The water-dripping apparatus is 

 a strong cylindrical tank of galvanized iron, two feet in diameter 

 aud eighteen inches in depth. The tank stands on three solid glass 

 supports set in feet made of mahogany wood ; the glass supports are 

 incased in cylinders of thick glass in which may be placed pumice 

 stone soaked in sulphuric acid in order to better the insulation. 

 From the tank a pipe projects about five feet and terminates in a 

 fine orifice ; the water flowing through the latter breaks into drops 

 immediately after leaving the nozzle of the tube. An insulated 

 wire connects the vessel with the electrometer placed in-doors, A 

 short time from the starting of the stream the can will be found to 

 be electrified to the same potential as the air at the point of the 

 tube. This potential is imparted through the conductors to the 

 electrometer, and a deflection of the needle ensues. 



Registering Apparatus. — The registering apparatus consists of a 

 powerful clock with weight, second pendulum, dial and gearing. 

 Tlie latter connects with, and turns a cylinder provided witli a pair 

 of thin brass bars furnished with spring hooks and hinges for hold- 

 ing the sensitized photographic paper. The cylinder turns once in 

 about thirty hours. The sensitized paper, or chart, is provided with 

 a zero line and a suitable scale division. On one side of the zero 

 line the positive potential is recorded, and on the other side the 

 negative. In front of the cylinder is a long cylindrical glass lens. 

 Near one side of the case covering the cylinder is a gas burner 

 with an opaque chimney carrying a round glass window on one 

 side. The pencil of light passes through the window, and also a 

 vertical slit in the case, falls upon the mirror connected with the 

 needle, and is reflected back upon the cylindrical lens, which 

 concentrates the rays in a point. This follows the motion of the 

 mirror and thus impresses upon the sensitized paper the curves 



