The theoretical sensitivity and response time of thermopile circuits are discussed, and Johansen's results are modified in the light of more approximate assumptions and rigid analysis. The practical design of thermopiles is considered, and it is concluded that a single element uncompensated pile is best for many purposes. The results are applied to the construction of thermoelectric cells, of which two types are described. In each of these the thermoelement is clamped in a copper case, which acts as a screen against stray radiation, as a reflector, and as massive cold junctions. The first type employs a bismuth antimony-cadmium alloy hot junction, the metals being made into wires by the Taylor process. The other type contains a bismuth antimony junction; the receiver is of thin gold leaf. The thermoelectric metals are distilled on to glass films half a micron thick, which support them. The second type is much stronger and quicker in response than the first, but is less sensitive. The cells are operated in a vacuum of 10-4 mm. of mercury, and a Dewar construction is employed for the vacuum container; this ensures steadiness in operation. A simplified form of cell dispensing with the copper case and employing a distilled element is described, which is suitable for many purposes. The use of these thermoelements as modified Pirani gauges is suggested. A typical bismuth antimony-cadmium alloy cell had a receiving area of 0.05 cm.2, a resistance of 8 ohms, and developed an E.M.F. of 55 microvolts for the radiation from a candle at a metre; its response time was 6 seconds. The other type had about one-third of this sensitivity, and a response time of 2.5 seconds.