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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 125 (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 125.  Current and voltage in series and parallel circuits. Figure 125(A) shows two resistors connected in parallel. Notice that the voltage across both resistors is equal. In figure 125(B) the same resistors are connected in series. In this case, the voltage across the resistors is not equal. If R_{1} represents a voltmeter, the only way in which it can be connected to measure the voltage of R_{2} is in parallel with R_{2}, as in figure 125(A).
LOADING EFFECT A voltmeter has an effect on the circuit being measured. This is called LOADING the circuit. Figure 126 illustrates the loading effect and the way in which the loading effect is kept to a minimum. Figure 126.  The loading effect. In figure 126(A), a series circuit is shown with R_{1} equaling 15 ohms and R_{2}equaling 10 ohms. The voltage across R_{2} (E_{R2}) equals 10 volts. If a meter (represented by R_{3 }) with a resistance of 10 ohms is connected in parallel with R_{2}, as in figure 126(B), the combined resistance of R_{2} and R_{3} (R_{n}) is equal to 5 ohms. The voltage across R_{2} and R_{3} is now 6.25 volts, and that is what the meter will indicate. Notice that the voltage across R_{1} and the circuit current have both increased. The addition of the meter (R_{3}) has loaded the circuit. In figure 126(C), the lowresistance meter (R_{3}) is replaced by a higher resistance meter (R_{4}) with a resistance of 10 kilohms. The combined resistance of R_{2} and R_{4} (R_{n}) is equal to 9.99 ohms. The voltage across R_{2} and R_{4} is now 9.99 volts, the value that will be indicated on the meter. This is much closer to the voltage across R_{2}, with no meter (R_{3} or R_{4}) in the circuit. _{} Notice that the voltage across R, and the circuit current in figure 126(C) are much closer to the values in 126(A). The current (I_{R4}) through the meter (R_{4}) in figure 126(C) is also very small compared to the current (I_{R2}) through R_{2}. In figure 126(C) the meter (R_{4}) 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? 
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