687 
atic: = 
one or more of the following difficulties: 
(1) lack of reproducibility, time of closing variable, 
voltage not always clean; 
(2) failure to remain in adjustment, necessitating 
cleaning of the contacts and "tinkering"; 
(3) tedious to use, requiring manual resetting. 
The most satisfactory semi-mechanical device of this kind used here 
employs the opening of a.relay contact, the relay being energized by a 
vacuum—tube circuit which also provides the synchronizing pulse for trigger- 
ing the time=base or brightening circuits, and a regulated voltage source. 
A stmple circuit found adequate at this laboratory is shown in Fig. 22, 
The relay-plaeed in the cathode of a flip-flop is normally energized. The 
trip pulse, after passing through 4 variable time-delay circuit, trips the 
flip-flop, de-energizing the relay, A Sigma type AH, 500-ohm plug-in relay 
was used here. 
Because of its simplicity, very little trouble has arisen with this 
type of generator. It was found necessary, however, to keep the voltage 
difference becween the relay coil windings and the relay frame as low as 
possible to eliminate electrolysis through the insulation. Furthermore, at 
recording speeds faster than 1 msec/in., even faster releasing relays ex- 
hibit some chatter and rounding of the step base. This makes difficult 
accurate measurement of the step height from photographic records in oscillo- 
scope calibration, However, at slower recording speeds, units of this type 
have performed quite satisfactorily. 
(ii) Electronic step-function generator. When the speed of recording 
is so fast that the chatter in a mechanical step-function generator cannot 
be tolerated, it is necessary to employ electronic means to generate the 
step function. The method developed here involves a rapid cutoff of the 
current to a voltage-regulator tube network such as described above, result= 
ing in a drop’in the output voltage which can appear as q positive or nega- 
tive step function, depending on the method of connection to the recording 
circuit, The step function generated in this manner is quite clean. 
The circuit developed at this laboratory is shown in Fig, 23. A type 
6AG7 tube supplies the powcr to the VR network, operating in push-pull with 
another 6AG7 to maintain a constant drain on the power supply. For step- 
function generation the 6AG7 supplying the VR network is abruptly cut off, 
