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).
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 (ER2) 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 (Rn) 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 (Rn) 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 (IR4) through the meter (R4) in figure 1-26(C) is also very small compared to the current (IR2) 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.
Q.30 What electrical quantity is measured by a voltmeter?
Q.31 How is a voltmeter connected to the circuit to be measured?
Q.32 What is the loading effect of a voltmeter?
Q.33 How is the loading effect of a voltmeter kept to a minimum?