VOLTMETERS

All the meter movements discussed so far react to current, and you have been shown how ammeters are constructed from those meter movements. It is often necessary to measure circuit properties other than current. Voltage measurement, for example, is accomplished with a VOLTMETER.

VOLTMETERS CONNECTED IN PARALLEL

While ammeters are always connected in series, voltmeters are always connected in parallel. Figure 1-25 (and the following figures) use resistors to represent the voltmeter movement.

Since a meter movement can be considered as a resistor, the concepts illustrated are true for voltmeters as well as resistors. For simplicity, dc circuits are shown, but the principles apply to both ac and dc voltmeters.

Figure 1-25. - Current and voltage in series and parallel circuits.

Figure 1-25(A) shows two resistors connected in parallel. Notice that the voltage across both resistors is equal. In figure 1-25(B) the same resistors are connected in series. In this case, the voltage across the resistors is not equal. If R₁ represents a voltmeter, the only way in which it can be connected to measure the voltage of R₂ is in parallel with R₂, as in figure 1-25(A).

LOADING EFFECT

A voltmeter has an effect on the circuit being measured. This is called LOADING the circuit. Figure 1-26 illustrates the loading effect and the way in which the loading effect is kept to a minimum.

Figure 1-26. - The loading effect.

In figure 1-26(A), a series circuit is shown with R₁ equaling 15 ohms and R₂equaling 10 ohms. The voltage across R₂ (E_R2) equals 10 volts. If a meter (represented by R₃ ) with a resistance of 10 ohms is connected in parallel with R₂, as in figure 1-26(B), the combined resistance of R₂ and R₃ (R_n) is equal to 5 ohms. The voltage across R₂ and R₃ is now 6.25 volts, and that is what the meter will indicate. Notice that the voltage across R₁ and the circuit current have both increased.

The addition of the meter (R₃) has loaded the circuit.

In figure 1-26(C), the low-resistance meter (R₃) is replaced by a higher resistance meter (R₄) with a resistance of 10 kilohms. The combined resistance of R₂ and R₄ (R_n) is equal to 9.99 ohms. The voltage across R₂ and R₄ is now 9.99 volts, the value that will be indicated on the meter. This is much closer to the voltage across R₂, with no meter (R₃ or R₄) in the circuit.

Notice that the voltage across R, and the circuit current in figure 1-26(C) are much closer to the values in 1-26(A). The current (I_R4) through the meter (R₄) in figure 1-26(C) is also very small compared to the current (I_R2) through R₂. In figure 1-26(C) the meter (R₄) has much less effect on the circuit and does not load the circuit as much. Therefore, a voltmeter should have a high resistance compared to the circuit being measured, to minimize the loading effect.