Electrical Engineering ⇒ Topic : DC Voltmeter

Ammeters and voltmeters are classed together because the principle of operation involved is the same for both the types of meters. An ammeter carries a definite fraction of the current to be measured, whereas a voltmeter carries a current proportional to the voltage to be measured. An ammeter is connected in series with the circuit whose current is to be measured. Insertion of any meter in the circuit should not change the circuit conditions and hence the resistance of an ammeter must be very small. On the other hand, a voltmeter is connected across a voltage source whose voltage is to be measured. The current drawn by the voltmeter must be as small as possible in order to keep the circuit conditions unaltered. Therefore, the resistance of the voltmeter must be very high compared to the resistance of the load connected across the circuit. Generally, the power loss in ammeters and voltmeters should be kept small. Suppose r_a is the resistance of an ammeter carrying a current of /₂ amperes, then the power loss in the instrument will be I. r_a. If this power loss must be small, then r_a must be small. On the other hand, if r, is the resistance of an voltmeter to which a voltage V is applied, then the power loss in the voltmeter will be      If this loss is to be small then r, must be very high.

A voltmeter is used to measure the potential difference between two points of a circuit. It is thus connected in parallel with the circuit or some part of the circuit as shown in Fig. (a). The voltmeter must have enough resistance so that it will not be injured by the current that flows through it, and so that it will not materially affect the current in the circuit to which it is connected. For this reason, a voltmeter is designed to have high resistance.

                                                       figure (a)

DC Voltmeter

To make the PMMC movement for reading voltage, multiplier resistance is connected in series with the meter so that when it is connected across the circuit, only the rated current of the meter passes through it. By connecting a number of resistances in series with a selector switch, the range can be increased to read different voltages.

Series resistance is given by

                                                                      ............. (1)