How to use and calibrate a DC voltmeter?

A DC voltmeter is made up of a DC ammeter connected in series with an additional resistor. Since the internal resistance of the ammeter is very small and can only withstand very low voltages, when using it to measure DC voltage, a high-resistance additional resistor (also called a voltage doubler) must be connected in series so that the additional resistor can withstand most of the voltage. The additional resistor is generally an internal type, but when measuring high voltage, an external additional resistor must be connected in series. The external additional resistor is supplied with the voltmeter, and the words "external resistor" are usually marked on the dial. The external resistor is an accessory of the voltmeter. Without it, the current passing through the voltmeter will be very large, which may burn out the instrument coil.

When connecting a DC voltmeter, pay attention to its positive and negative poles. Usually, there are "+" and "-" marks next to the terminal of the voltmeter. The "+" (positive end) of the terminal is connected to the high potential of the voltage being measured, and the "-" (negative end) of the terminal is connected to the low potential of the voltage being measured, as shown in Figure 11-2. The positive and negative poles must not be connected incorrectly, otherwise the pointer will reverse and may be bent.

The DC voltmeter with calibration limit below 200V can be wired as shown in Figure 11-3.