044 



ELECTRICAL MEASUREMENTS 



the strips will vibrate with a frequency twice that of the voltage. 

 One of the condensers, Ci, C*, is then adjusted until this deflec- 

 tion disappears. If the adjustment is not perfect, the two halves 

 of a symmetrical alternating-current wave will appear dissimilar. 

 If the connection at k is not at the correct point, the wave will 

 be shifted with respect to the zero line. 



The metallic vibrator case should be connected to d. The 

 sensitivity can be varied by altering B. 



(WWsw 



FIG. 405. Connections for determining the wave form of a small current 

 by electrostatic oscillograph. 



The sensitivity attained is as follows: with B = 300 volts, 

 e = 2,000 volts, effective, scale distance 70 cm., the amplitude 

 of wave trace is 2 cm. 



For the measurement of very small currents the connections 

 shown in Fig. 405 may be used. The voltage B is made very 

 large by using either a high-tension battery or two condensers 

 in series which are continuously charged from an influence 

 machine. 



FIG. 406. Braun tube for determining wave form. 



The Braun Tube. The Braun tube 7 is a form of vacuum tube 

 especially designed for determining wave form. 



Fig. 406 gives an idea of the tube as now made. The alu- 

 minum cathode is at K, the anode at A; they should not be less 

 than 15 cm. apart. At T is a grounded brass target, pierced 

 by a small hole about 0.5 mm. in diameter. The glass screen. 

 S } is covered with zinc sulphide, or calcium tungstate. 



