Abstract
The wattmeter depends for its action on the torque exerted by the electromagnetic field in a waveguide on a vane suspended inside the guide by means of a fine quartz fibre. The deflection of the vane can be measured by means of a small mirror attached to it and a lamp-and-scale arrangement. The specific couple of the quartz fibre being known, the torque can be calculated. The relationship between power and torque can be determined by an auxiliary experiment in which nodal positions of certain standing-wave patterns are determined. The theory of this method of calibration is based on the theorem of adiabatic invariance of action and has been explained in a previous paper. The intrument will measure microwave power with an error not exceeding about ± 1½%. The measurement is absolute, depending only on measurements of mass, length and time, and the amount of power absorbed is insignificant.
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