A net radiometer designed for optimum sensitivity and a ribbon thermopile used in a miniaturized version

The difference between emf output E and electric power output N of net radiometers using thermopiles made by copper deposition on constantan wire is discussed and formulas are given for both. The mathematical treatment shows that the number of thermojunctions should not be smaller than that required for maximum power output; however, somewhat larger numbers can be tolerated. Maximum power output N0 is shown to be achieved (a) when the thermal conductance of the thermopile itself equals all the remaining conductances between the sensitive surfaces, GΣ, and (b) when GΣ is a minimum. N0 does not depend on the actual wire size or thermopile dimensions but E does depend on these quantities. With optimum design the copper deposit is only 8 to 11 per cent of the cross-sectional area of the constantan wire. The formulas are easily adapted for radiation instruments in general. Applications are given, and a miniaturized net radiometer, based on a ribbon thermopile permitting a high concentration of junctions in a given space, is described. This instrument, being only 1.1 cm in diameter, exhibits a sensitivity comparable to that of much larger instruments and is ideally suited for measurements of net radiation of botanical or zoological surfaces (e.g., individual leaves).