The characteristics of electromagnetic instruments can be described from two aspects: scale and frequency characteristics, as follows:
In terms of scale, its deflection angle changes with the square of the measured DC current or the square of the measured AC current effective value, so the scale has a square law characteristic; when the measured value is small, the division is very dense, and the reading is difficult and inaccurate; when the measured value is large, the division is sparse, and the reading is easy and accurate.
In terms of frequency characteristics, the magnetic field is established by the fixed coil passing the current. In order to measure higher voltages without causing the measuring mechanism to exceed the allowable current value, the fixed coil has more turns, larger internal resistance, and larger inductive reactance, which changes with the frequency, thereby affecting the accuracy of the instrument.
Therefore, electromagnetic instruments are only suitable for circuits with frequencies below 800Hz.
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