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Similarly, on eliminating Va between equations (xi) and (xii), we find that :-
where K 2 is also a constant dependent upon circuit resistances and valve characteristics only.
Substituting this value in equation (ix) we have :-

where K is a constant and, from equation (v) we see, on substituting the values of equations (xiii)-(xv), that the
condition that the output voltage (V out) shall be independent of the supply voltage at the working point is :-

This equation of balance is seen to involve circuit resistances and valve characteristics only.

REFERENCES properly adjusted, such that a positive galvanometer
deflection denotes an increase in anode current; the
(1) "Measurement of Small D.C. Potentials and Currents in deflection passes through a minimum as the filament
High Resistance Circuits by using Vacuum Tubes" current is varied. At a minimum the deflection is
independent of small changes in filament current.
by W. B. NOTTINGHAM. Jour. Franklin Inst. Vol. 209,
p. 287 (1930). (6) "A Balanced Electrometer Tube and Amplifying Circuit
The grid current and anode current characteristics for Small Direct Currents"
of vacuum triodes are discussed and the importance by G. P. HARNWELL and S. N. VAN VOORHIS. Rev.
stressed of the effect of the grid current on the equa- Sci. Inst., Vol. 5 (1934).
tions for sensitivity. The simple equations of the d.c.
amplifier are developed and three types of output An extension of the DuBridge circuit is described
circuits are dealt with. Complete circuits for the which allows valves of a fairly wide range of charac-
measurement of potentials and currents by direct de- teristics to be used. A very stable direct-current
flection and null methods are discussed. two-tube amplifier is described which may be used
independently for the measurement of currents of the
amps or in the output of an electrometer
order 10
-12
(2) "A Compensated Thermionic Electrometer" valve circuit.
by K. G. COMPTON and H. E. HARING. Trans. Electro-
chem. Soc. Vol. 52 (1932. (7) "Direct-current Amplifier Circuits for use with the
Here a compensated single tetrode electrometer circuit Electrometer Tube"
is described and its operating principles are discussed. by D. B. PENICK. Rev. Sci. Inst. 6, 115 (1935).
The circuit compares favourably with “balanced valve” This shows an improved circuit for use with the
circuits both as regards stability and sensitivity. The single tetrode type of valve. A concise analysis is
degree of compensation has been found adequate to given of the six circuits proposed for using screen-grid
make possible the use of dry batteries or even properly valves in a direct type of amplifier without resorting
controlled rectified alternating current as a power
source to the directly heated electrometer valve de- to the more expensive and elaborate bridge type. The
scribed. A bridge type circuit is outlined which is balance equations are stated for the most generallv
useful circuit. The stability of the circuit is discussed,
capable of measuring currents less than 10 -17 amp by and a convenient method for obtaining a balance by
the “rate of drift” method. experimental methods is suggested.
(3) "An Improved D.C. Amplifying Circuit" (8) "The Measurement of Direct Potentials originating in
by L. A. DUBRIDGE and H. BROWN. Rev. Sci. Inst. Circuits of High Resistance"
Vol. 4, p. 532 (1933). by R. H. CHERRY. Trans. Amer. Electrochem. Soc.
This circuit compensates simultaneous changes in 72, pp. 33-44 (1937).
emission and anode and grid voltages, by balancing
the effect of changes in the anode current against the Various methods of detecting direct potentials in
effect of changes in the current to the space-charge circuits of high resistance are discussed. A simple
grid. Changes in the emission while the anode and single triode circuit is described which is capable of
grid voltages remain constant are not compensated. detecting potentials to within 0.1 millivolt in circuits
of approximately 10 ohms resistance. This amplifier,
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(4) "The Application of the F.P.54 Pliotron to Atomic in which controls have been reduced to a bare mini-
Disintegration Studies" mum, is suitable in addition to ordinary potential
by L. R. HAFSTAD. Phys. Review, Vol. 44, p. 201 measurements, for the measurement of small direct
currents such as photoelectric currents, diffusion cur-
(Aug., 1933). rents in polarographic studies, and condenser leakage
The single tube circuit used by DuBridge was chosen currents.
since its sensitivity is inherently twice that of the
balanced circuit. Circuit details, drift figure, and the (9) "An Electrometer for Measurement of Voltage on Small
ionisation chamber and electrometer mountings are Ionisation Chambers"
fully explained. by F. T. FARMER. Proc. Physical Soc. LIV, p. 435
(5) "Uber ein Neuartiges Rohrengalvanometer" (1942).
This describes an instrument of exceptionally high
by VON G. BARTH. Zeits. f. Physik. 87, 399 (1934). input resistance combined with low input capacity. It
This is one of the most satisfactory circuits designed employs an electrometer valve in series with a high
to eliminate the effect of drift in battery e.m.f. and impedance pentode and the two valves together function
changes in filament emission. When the circuit is in the manner of a “cathode follower”.
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