643 
are 
against time, photographic recording of the spot is made, The spot may be 
moved across the screen at a uniform rate and a still picture taken, or the 
film may be moved past the image of the spot at constant spced, If a linear 
sweep (time base) is used to move the spot, the starting of this tracer must 
be synchronized properly withthe event to be recorded by some trigger signal; 
it is convenient in some cases to add a controlled interval between the two 
by a time-delay circuit. Further, it is nearly always necessary to increase 
the cathode-ray spot intensity for a suitable time interval and again turn 
it off, because mechanical camera shutters are usually too slow and inaccu- 
ratee Some sort of calibration of the speed of the spot or film is also 
necessary and can be supplied to the tube from a reference oscillator either 
as a deflection signal or as a modulation (dotting) of the spot intensity. 
Auxiliary equipment is also necessary to provide suitable power for 
operation and, if much routine recording is to be done, master control 
(switching) circuits for the various operations are a great time saver. 
It should be noted that the equipment described in this report was de- 
veloped for use in the underwater explosion research at this laboratory. The 
instrumentation for air-blast work is to be described separately in NDRC Re- 
port A=373 (OSRD-6251). 
