Electrical Oscillations in Air. 327 



marked spiral line of over fifty turns. The two sparks from 

 the Ruhmkorff-coil left their trace upon the blackened paper 

 as spires, which therefore measured the number of revolutions 

 of the cylinder between the swings of the pendulum, and thus 

 gave the rate at which the mirror was revolving. The chrono- 

 graph record enabled us to measure the time to -g^ of a 

 second. 



In any operation which requires that an electrical spark 

 should make a record upon a disk or cylinder revolving at 

 great speed a large Ruhmkorff-coil and a strong battery are 

 necessary, especially if the primary circuit of the Ruhmkorff- 

 coil is broken by a pendulum. With the ordinary automatic 

 break, such as is commonly employed upon induction-coils, 

 the failure of a single break is unimportant. If, however, a 

 single break fails when a pendulum-break is employed, the 

 record of the experiment is an imperfect one. An excess of 

 battery-power and a large battery are therefore necessary. A 

 metallic breakpiece also was found to be more inconstant for 

 our purposes than a mercury -break. 



It was found that a certain simplicity of contrivance was 

 necessary in the method of discharging the air condenser 

 through the inductance-circuit. No arm connected with the 

 revolving mirror could be trusted to break or make an elec- 

 trical circuit by throwing in or out any form of switch. The 

 great speed at which it revolved broke all arrangements which 

 were tried. By placing a short stiff brush of minute size upon 

 the end of the flying terminal connected with the revolving 

 mirror, and allowing this brush to rub against a brass plate 

 set in an ebonite circle of 41 centim. in diameter, constancy 

 of action was secured. 



In order to obtain the same difference of potential at each 

 run, experiments were first made with various forms of unit- 

 jars and pith-ball electrometers. These devices were speedily 

 given up in favour of a simple balance-electrometer. One of 

 the pans of a delicate balance was replaced by a metallic disk. 

 A similar disk, which was stationary _, was placed imme- 

 diately below the movable one. By properly weighting the 

 remaining balance-pan great delicacy and range of indication 

 were obtained. This apparatus constitutes, in fact, an abso- 

 lute electrometer. A suitable guard-ring can be placed around 

 the movable disk. 



When the air condenser had been charged to a definite 

 potential, the movable disk of the electrometer closed an 

 electrical circuit in which was included an electrical bell. 

 The observer stationed at the chronograph, at the instant he 

 heard the bell, drew the carriage connected with the stylus 



