Correction of frequency errors in wattmeters

MOST commercial wattmeters have a guaranteed limit of error of 1/4 per cent of full scale deflection on frequencies up to 133 cycles per second, but due to inherent errors this accuracy is not maintained at higher frequencies or with very low power factors. The errors referred to may be attributed to two things: the self-reactance of the voltage circuit, and the mutual reactance between the voltage coil and the current windings.