When measuring current, the ammeter should be connected in series with the circuit under test; When measuring voltage, the voltmeter should be connected in parallel with the circuit under test. When measuring DC current and voltage, attention must be paid to the polarity of the instrument, and the polarity of the instrument should be consistent with the measured polarity. Measurement of high voltage and high current. When measuring high voltages or currents, voltage transformers or current transformers must be used. The range of the voltmeter and ammeter should match the secondary rating of the transformer. The general voltage is 100V and the current is 5A.

An ammeter is made according to the action of a magnetic field force on an energized conductor in a magnetic field. There is a permanent magnet inside the ammeter that generates a magnetic field between the poles. In the magnetic field, there is a coil with a hairspring at each end of the coil. The springs are connected to a terminal post of the ammeter, and between the spring and the coil, there is a rotating shaft connected. There is a pointer on the rotating shaft relative to the front end of the ammeter. Pointer deflection. Because the magnitude of the magnetic field force increases as the current increases, it is possible to observe the magnitude of the current through the deflection of the pointer. This is called a magnetoelectric ammeter, which is the kind we usually use in the laboratory.