OSCILLOGRAPHS 



389 



of information may be used with a continuous-film oscillograph with 

 the usual form of optical system. The film is run at very slow speed 

 in order to obtain normally an envelope of the wave. At intervals 

 the speed of the film is increased to a value sufficient to resolve the wave 

 and show the actual wave shape. A record of the time may be made on 

 the film by photographing a clock at regular intervals, say once a 

 minute. A record made in this way is shown in Fig. 19, No. 6. 



Fig. 21 — Amplitude calibration of polar oscillograph. 



Frequently there is an advantage in recording two or more variables 



simultaneously. A record obtained with two vibrators is shown in Fig. 



19, No. 7. 



Performance 



The limitations of an oscillograph lie mostly in the vibrator and, to a 

 smaller degree, in the optical system and photographic emulsion used. 

 The frequency characteristic of the vibrator up to 800 cycles is quite 

 uniform, permitting records of disturbances having components in this 

 range to be made with little distortion. This range includes the first 

 13 harmonics of 60 cycles and the first 32 harmonics of 25 cycles. 



The vibrator may be wound to have any impedance in a wide range. 

 If wound to have a high impedance it is especially suited for recording 



