686 
Stas 
(ii) Gaseous voltage-regulator tubes. If a gaseous voltage-regulator 
tube is operated under constant current, its terminal voltage will remain 
quite constant. The amount of stability is a function of the type and the 
individual tube, It has been found possible to select type VR105 tubes with 
a stability better than 0.1 percent, The noise level is less than 1 mve 
For selection the tubes are warmed up for one half hour, under their 
normal operating conditions, before being tested. Instability exhibits 
itself in rapid fluctuations during continual operation or in voltage jumps 
after the current is interrupted momentarily. Usually any voltage change 
is accompanied by a shift in the glow discharge over the cathode. 
The circuit employed in this laboratory is shown in Fig. 21, It operates 
from an ordinarily filtered power supply. One voltage-regulator tube, type 
VR150, is employed to filter out any supply variations, thus maintaining 
constant current through the VR105. This circuit will provide adequate 
power for most calibrations as well as maintain accurate voltage. 
Bt Ry p Ro 
I YA 
" \ 
17 ma 10 ma \ | 
‘ a oF 
VR 150 : = r 
fp mae d dnd det dnarns 
~ Ry, Ro Function of load current 
Fig, 21. High-current standard-voltage source. 
(ob) Step-function generators. -- (i) Relay step-function generator. 
Although it is possible to obtain clean voltage changes by mechanical switch- 
ing, the mechanical devices tried here have always suffered to some ectent from 
