Copper-oxide rectifiers in ammeters and voltmeters

This paper deals with various errors that may arise when a moving-coil instrument is used in conjunction with a copper-oxide rectifier to measure alternating currents and voltages. The effects of circuit impedance upon the reading of a rectifier ammeter and of the amount of series resistance upon the reading of a rectifier voltmeter are investigated. It is also shown that the effective resistance of a rectifier as calculated from tests with different series resistances varies considerably, owing to the distortion of the current wave even when the voltage is sinusoidal. The use of rectifier instruments having linear characteristics to measure very bad wave-forms, such as telephonic currents, is dealt with; and it is shown that such instruments, calibrated with a sine wave, read the effective value within about 10 per cent for any shape of wave, however much it may be distorted. Finally, the difficulties encountered in the application of rectifier instruments to current transformers are examined and methods of reducing those difficulties are suggested.