NATURAL OBSERVATION LIMIT OF RADIOMETRIC MEASUREMENTS

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Abstract
The sensitivities of thermocouples, radiomicrometers, bolometers, expansiometers and radiometers are compared by determining the minimum amount of energy that can be detected by a single observation. The instruments are considered to be critically damped and the accuracy of the measurements to be limited by the Brownian motion fluctuations of the measuring device. A detectable deflection is arbitrarily taken as four times as large as the Brownian motion fluctuations. The theoretical considerations are included for their generality or for their direct bearing on the practical design and use of the instruments. A thermocouple with a galvanometer or a radiomicrometer is the most sensitive of the measuring instruments considered. By a single observation with a galvanometer of one sec. period, 3×10−11 gm·cal./sec. falling on a thermocouple receiver 1 mm2 should be detected. A complete theory of the vacuum bolometer is presented. The ideal sensitivity of a vacuum bolometer is approximately the same as for a thermocouple but practical difficulties make the bolometer less sensitive. The vacuum tube is less sensitive than a galvanometer for use with a thermocouple or bolometer. A thin bimetallic receiver would not be as sensitive as a thermocouple for making accurate radiometric measurements. A vacuum thermocouple is about 20 times more sensitive than a radiometer mostly because the gas pressure that is needed for the action of the radiometer only allows the receiver to be heated about 1/10th as high as a vacuum thermocouple receiver. The sensitivity of a critically damped galvanometer or radiometer, limited by the Brownian motion, is proportional to the square root of the period.