A recording system is described which measures the electromotive force generated by slowly responding linear‐type thermopiles. A means is provided for compensating electrically for the zero signal drift so that the thermopile may be used conveniently under rather unstable thermal surroundings. A twin phototube arranged in a feedback type amplification circuit is used as a receiver for the light beam reflected from the mirror of a sensitive galvanometer connected in series with the thermopile. The current from the feedback circuit which automatically keeps the galvanometer essentially at its zero signal position is directly proportional to the thermopile signal. This current may be measured by a conventional microammeter or an automatically recording meter if desired.

Topics
Thermopile, Phototubes, Galvanometers, Microammeter, Electromotive force
1.
P. A.
Leighton
 and
G. S.
Forbes
,
J. Am. Chem. Soc.
51
,
3551
(
1929
).
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2.
R. P.
Teele
 and
S.
Schuhman
,
Natl. Bur. Standards J. Research
22
,
431
(
1939
).
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3.
The authors wish to thank Professor J. E. Gullberg, Zoology Department, University of California, for the four‐terminal manganin resistor similar to the one described by Teele and Schuhman.
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© 1953 The American Institute of Physics.
1953
The American Institute of Physics
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