Abstract
A PAPER by G. F. Gainsborough entitled "Experiments with Thermocouple Milliammeters at Very High Radio Frequencies" (J. Inst. Elect. Eng., 91, Part III, No. 15; Sept. 1944) describes work conducted at the National Physical Laboratory under the auspices of the Radio Research Board. In order to assess the performance of commercial thermocouple milliammeters at frequencies up to 700 Mc./s., a reference standard air-milliammeter was first developed by the author, following principles first described by J. A. Fleming in 1910. Each of two similar air cells connected by a capillary tube with a liquid index contains a resistive wire which can be heated either by an alternating or direct current. With a capillary tube of 1 mm. bore, and using a low-power microscope to observe the index, the apparatus described in the above paper gave readings of current of the order of 10 mA., which could be reproduced with an accuracy of 1 part in 1,000. The sensitivity could be altered by using capillaries of different bore, and filaments of different resistance.
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