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The paper deals with the application of the Hall effect in a semi-conductor, like germanium, to the measurement of power transmitted along a line or a hollow metal waveguide. It is shown that the expression for the instantaneous value of the Hall e.m.f. generated in a small piece of semi-conductor erected in an electromagnetic field is of exactly the same form as the expression for the instantaneous power traversing the semi-conductor. Thus the mean value in time of the Hall e.m.f. is a direct measure of the power, and this is true for alternating, pulsating or steady fields. The paper describes various types of wattmeter embodying the foregoing principle and applied to a wide range of frequencies. The method is particularly suitable for use when strong magnetic fields are available, and heavy-current systems consequently lend themselves to the application of this device without the need for current transformers. Experiments at 50c/s and again at 300Mc/s with an n-type germanium crystal erected in the annular space between the inner and outer conductors of a coaxial line showed that, for a given power, the Hall e.m.f. was the same at the two frequencies, within the accuracy of the measurement. Thus there is a prospect of calibrating such an instrument at a low frequency and using it, if desired, at a very high frequency.