When a semiconducting crystal is mounted in the path of an electromagnetic wave, a Hall e.m.f. is set up in the material along the direction of propagation with a value proportional to the power transmitted. The device operates most satisfactorily when the wave impedance is small, so that for a given power the electric-field component is reduced to a minimum. The use of a resonant cavity with the crystal erected in it at a point of strong magnetic field enables such favourable conditions to be established, and this arrangement forms the basis of the wattmeter described. The instrument is capable of measuring power at 4 Gc/s with an error of ±3% from 30 mW to about 20 watts, and with the unique feature that this performance can be achieved at any standing-wave ratio between unity and 0.1 while absorbing only about 3.4% of the power measured.Some experiments on semiconductors mounted directly in a waveguide are also described, showing the possibility in this case of using several crystals in cascade.