The measurement of power at a wavelength of 3 cm by thermistors and bolometers

A description is given of measurements made on several thermistors and bolometers at wavelengths of 3.18 and 3.26 cm, using a water calorimeter to investigate their performance as indicators of absolute power. The calorimeter, details of which are given, was designed to measure powers of 2-5 watts; and by means of a 35 dB directional coupler known absolute powers at the milliwatt level were delivered to the thermistor and bolometer mounts. Values of efficiency, defined as the ratio of the power measured in a d.c. calibration to the net input power, of 0.87 to 0.91 were found, with no significant change on varying the operating resistance of the thermistors and bolometers over the range 150-300 ohms.