A milliwattmeter for centimetre wavelengths

The instrument is in the form of a differential air thermometer, and consists of two similar glass cells connected by a glass capillary tube. A carbon-coated strip is placed in each of the two cells; one of these strips absorbs the radio-frequency power to be measured and the other is heated by direct current. A liquid pellet in the capillary tube is displaced as a result of the differential expansion of the air in the two cells arising from the heat dissipated in the carbon-coated strips. After balancing the system by adjustment of the d.c. cell the radio-frequency power is finally determined by the measurement of the d.c. power which must be substituted for the radio-frequency power in the same cell in order to maintain the state of balance. The instrument has been used to measure powers of 10 to 100 mW at a wavelength of 3cm with a discrimination of about 0.2 mW.