697 
BRS 
Ry Rg/(RgCu)*, 
2 
C, = 1f0°L - Cy = 20. 
If an unknown impedance is connected in parallel with C5, its capacitance 
Cy is equal to the amount by which the condenser OC, must be decreased to 
rebalance the network, and its resistance R, is determined by the change in 
R, necessary for rebalance. These two balances are independent. In the 
design used, a nominal driving frequency of 10 kce/sec is employed, and 
capacitance ranges of 0 to 100,000 pf, O to 10,000 ppt are obtained by 
suitable values of inductance L, 
The driving frequency is provided by a resistance=stabilized oscillator, 
the output of which is amplified in a tuned L-C circuit driven by a cathode 
follower, and coupled to the T=network by a second cathode follower, At 
balance, no signal appears at the output of the network, and this balance 
condition is determined by an electron-ray (magic-eye) tube. This tube is 
operated with its triode amplifier section biased to cutoff; an a-c signal 
applied on the grid produces a dec voltage on the ray-control electrode 
which changes the shaded area on the fluorescent target. For greater 
sensitivity a cathode-ray oscillograph or other indicating instrument may be 
connected to the terminals provided. 
The T=network used has the advantage, as compared to a bridge circuit, 
that both input and output voltages and the unknown impedance have one side 
grounded. The balance is, however, frequency dependent; hence a stable 
oscillator frequency with pure wave form must be provided to prevent drift 
and balance errors from harmonic frequencies passed by the network. Good 
quality air and mica condensers should be used in the network. 
The balance drifts while the oscillator is warming up and is affected 
by large line-voltage variations, but these errors are usually not serious. 
Except fer condenser drift or failure, the accuracy of capacitance balance 
is 0.1 percent of full scale. Units of this and earlier types have been in 
service for two and one-half years with satisfactory results. 
(b) Q-step calibration. -- The Q-step calibration procedure is, in 
effect, a means by which a known quantity of charge can be applied to a 
