675 
sve 
The delay pulses may be taken from terminals on the front panel of the 
unit. The same pulses are fed into a mixer which gives a square beam-brighten- 
ing pulse whose delay is controlled by the first timing olrcuit and whose 
duration is controlled by the second timing unit, Exaept fay leakage in the 
timing condensers, this circuit has caused no trovble over an operating period 
of a year. The aceuracy of timing, when checked, has bcen better than 1 per- 
cents 
(c) Trigger-circuit beam brightener, -- The purpose of this unit is to 
turn on the beam of a cathoae-ray tube upon the reception of a positive pulse 
(any duration) and turn it off when a negative pulse is received. Operation 
from 1-msec duration to infinite duration is possible. The unit (sec Fig, 18) 
is part of a complete oscillograph, but the description is included here as 
an example of a useful method, In practice it is activated by the time delay 
and beam brightener described in the previous section. 
The operation is based on a flip-flop type of circuit with two stable 
conditions. A positive pulse of short (or long) duration makes the first 
tube conducting and the second tube nonconducting. The positive swing of the 
second plate is applied to a level shifter and thence to a cathode follower 
for the output controls, A positive pulse returns the circuit to its original 
condition, As the whole unit is operated at the level of the cathode-ray tube 
cathodes, this makes a convenient way to control the electron-beam intensity. 
This unit has been in use for approximately six months and has required 
readjustment only when the 6SN7 tube was changed. 
(a) Synchronization circuits. -- (i) Use of firing line. It is evident 
that the voltage required to fire an electric detonator cap provides a signal 
related te the time of explosion. In order to make use of this signal several 
difficulties must be kept in mind. First, a delay of several milliseconds 
occurs between application of the firing voltage and detonation of the cap. 
This interval may or may not be reproducible, depending on the particular cap 
used and the current in the circuit. If this variable delay is unimportant, 
a simple switch arrangement can be used to provide the desired pulse. One 
precaution must often be observed, namely, that the firing line be left un- 
grounded in order to prevent ground loops with the signal cables, These, if 
present, may induce considerable signal because of fluctuations in the firing- 
line current. Because of this, it is usually necessary or desirable that the 
