The design criteria for fast thermopiles of maximum sensitivity are obtained, with particular reference to their use with interrupted radiation in infra-red spectroscopy. It is shown that since the heat capacity of short, fine wires can be made small compared to that of the receiver, the conventional construction has certain advantages over evaporated thermopiles. Sources of noise are discussed and it is shown that in the cases treated the noise caused by the temperature fluctuations of the receiver is somewhat less than that resulting from Johnson noise. If Johnson noise is the chief factor, the minimum detectable power in the form of incident radiation is inversely proportional to a quantity S=E/Rl (E=voltage sensitivity; R=thermopile resistance) defined as the absolute sensitivity. The optimum design of d.c. thermopiles is derived and a factor of merit for thermoelectric materials, Q/(Kp)! (Q=thermoelectric power; «=thermal conductivity; p=electrical resistivity) is obtained. Physical constants and factors of merit for various materials are tabulated. Cd-Sb and Te alloys have the highest factor of merit but it is concluded 6 L. C. Roess, Rev. Sci. Inst. 16, 172 (1945).
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