ALTERNATING-CURRENT INSTRUMENTS 75 



driven by a motor. The mirror M is then pulled up out of the 

 way and a mechanism causes tho shutter to open and close dur- 

 ing one revolution of the drum. In this case the time axis is 

 furnished by the movement of the film. 



The oscillograph vibrators are connected into the circuit 

 in the same manner as direct-current ammeter and voltmeter 

 coils are connected, Fig. 72. As the current vibrator can carry 

 but a small current, about 0.1 amp., it is connected in parallel 

 with a non-inductive shunt which is in series with the line. The 

 voltage vibrator is connected across the line in series with a high 

 non-inductive resistance. The current vibrator will then vibrate 

 with an amplitude proportional to the circuit current and in 

 phase with it. The current through the voltage vibrator will be 

 proportional to the circuit voltage and in phase with it. 



