Power-circuit instruments for the higher range of audio frequencies

The extending use being made of frequencies from 900 to 12,000 for special purposes such as induction furnaces, etc., introduces new problems in instrument design and calibration. Previous practical a-c instruments were either of the electrodynamic or moving iron types designed for low power frequencies or thermal and rectifier types for audio and radio frequencies, these latter usually being confined to measurements of only current and voltage. The ordinary low frequency electromagnetic instruments have such high coil reactance and losses that they are unsuitable for the higher power frequencies in question without special arrangements, which are described in the paper. Furthermore, the usual calibration methods and standards as previously used for low frequencies are not adequate, and therefore, new methods of calibration were introduced.