Abstract :
The wattmeters described may be regarded as typical instruments, and depend for their operation upon measurements of the Hall e.m.f. produced by a germanium crystal when placed in a magnetic field. The crystal carries a definite fraction of the load current and the magnetic field is made proportional to the supply voltage. The power-frequency instrument for use up to 150 c/s incorporates an iron-core magnetizing coil, while the audio-frequency design, capable of operation up to 20 kc/s, employs only air cores and includes careful screening arrangements. Experience has shown that high-permeability materials must be avoided for precision measurements of power by this means, except over a comparatively narrow frequency band. The audio-frequency instrument, which gives about 2 cm deflection for 50 mW, is shown to read 3.4% low at 15 kc/s and 5.3% low at 20 kc/s.
