670 
Set cs 
The "level=shifter" circuit just described has the disadvantage of 
drawing current from the high-voltage supply. Its operation is also criti- 
cal to the grid potential of tube To. Small variations due to resistor 
changes may thus cause drift in intensity over a period of time. A coupling 
method which is preferable for high accelerating voltages consists in putting 
the whole brightener circuit and its power supply at the potential level of 
the cathode=ray tube grid, With such a circuit the only coupling to circuits 
at ground potential required is for synchronizing pulses, which can be ap- 
plied by a low-capacity condenser, and the power required is obtained from 
a source independent of the accelerating voltage supply. A circuit of this 
kind is described subsequently [Sec, 3(c)]. 
3. Time bases and synchronization circuits 
In any recording of transients it is necessary to switch on the cathcde- 
ray tube spot for a controlled interval after application of a signal which 
is properly synchronized with theevent to be recorded. If time resolution 
is obtained by moving the cathode-ray tube spot across the screen, it is 
necessary also to provide a properly synchronized linear single-sweep cir- 
cuit. In many cases it is desirable to provide a controlled time delay be- 
tween the synchronizing pulse and the operation of these circuits. The time- 
delay circuit should provide accurately known intervals, the brightening cir- 
cuit should provide uniform intensity of the electron beam, the sweep circuit 
should displace the trace at a constant rate across the screen, and all units 
should trigger reliably on readily obtainable pulses. The circuits which 
have been developed to meet these needs are described in the following. 
An important practical consideration in underwater-explosion recording 
is the means of obtaining a synchronization pulse. The simplest solution is, 
of course, to eliminate the need of accurate synchronization by use of a con- 
tinucus-film movie camer2, which is started manually just before the time of 
interest. This method has also the advantage of being able to give a virtu- 
ally unlimited time duration, but if any very great time resolution is re- 
quired, large quantities of film are required, with increased expense and 
problems in film processing as 2 resulte In some cases, however, use of such 
a camera may be the best solution. 
