OSCILLOGRAPHS 379 



With an oscillograph of this type it is not practicable to allow the 

 film to be moving all the time on account of waste of film and the 

 maintenance difficulties. In order to avoid this and still make it 

 possible to begin recording soon after the beginning of a disturbance, 



Fig. 12 — Quick acting magnetic clutch on continuous-film oscillograph. 



the motor and associated gears are left running the whole time during 

 which a transient may be expected, and, when a disturbance occurs, a 

 quick acting magnetic clutch engages the film driving shaft with the 

 motor which puts the film in motion very quickly. The line relay 

 lights the oscillograph lamp at the same time. The whole recording 

 mechanism may be put in operation within 0.02 second, thus insuring 

 a good record of any but a very short transient. 



Normally the lamp would require several hundredths of a second to 

 become lighted to full brilliancy if operated at normal voltage. How- 

 ever, with a voltage several times normal and by the use of the circuit 

 shown in Fig. 11, it is possible to bring it to full brilliancy within 0.01 of 

 a second without danger to the lamp. When the circuit is closed by 



