When measuring current and voltage, the correct selection and use of ammeters and voltmeters not only has a direct effect on the accuracy of the measurement results, but also on the safety of the operator and the life of the instrument. This means a comprehensive and targeted selection.

(1) Selecting the type of instrument When measuring DC current and voltage, a magnetoelectric instrument should be selected. When measuring AC current and voltage, an electromagnetic system instrument or a rectifier system instrument should be selected. If higher accuracy is required, the electrical system instrument can be selected. When it is necessary to use both AC and DC, the electromagnetic system instrument for both AC and DC can be selected; in cases where higher accuracy is required, the electric system instrument can be selected.

(2) When selecting the accuracy of the instrument as a standard measuring instrument or a precision measuring instrument, the instrument of class 0.1 or 0.2 can be selected; the instrument of class 0.5 or 1.0 can be selected in the laboratory; the instrument of class 1.5 or less can be selected for general area measurement. For additional devices used in conjunction with the meter, such as shunt resistors, voltage divider resistors, instrument transformers, etc., the accuracy level should be 2 to 3 stops higher than the accuracy level of the meter itself to ensure the accuracy of the measurement results.

(3) After selecting the internal resistance of the meter and connecting the meter to the circuit under test, the power consumption of the meter itself should be minimised so as not to affect the original working condition of the circuit. Therefore, when selecting the internal resistance of the instrument, the following should be observed: the internal resistance of the voltmeter should be as large as possible; the internal resistance of the ammeter should be as small as possible.

(4) Selecting the range of the meter In actual measurement, to minimise the measurement error and ensure the safety of the meter, the range of the ammeter and voltmeter should be selected according to the following principles:

The selected range should be larger than the measured range.

②Select the range to be measured within more than two-thirds of the full scale of the instrument.

If the size of the measurand cannot be estimated, the maximum range of the instrument should be selected for testing and then gradually changed to a suitable range.

(5) Selecting the working conditions of the instrument Portable instruments are generally selected for instruments used in the laboratory; built-in instruments should be selected for instruments used in switchboards or electrical equipment panels. If there are special requirements for ambient temperature, humidity, external electromagnetic field and other conditions, the selection should be made according to their requirements so as to minimise the additional error of the instrument.

(6) Selecting the Insulation Strength of the Instrument When selecting the instrument, the insulation strength of the instrument and additional equipment should be determined according to the voltage of the circuit under test, so as to prevent accidents that endanger personal safety and damage the instrument.