Two characteristics of circuits must be remembered in using voltmeters: (1) Voltage is the measurement of a potential between two points; so the reading on a voltmeter is the difference between the connection points. To reduce the effect of adding a voltmeter to the circuit, the meter is constructed with a very high internal resistance. (2) Anytime that electricity flows through a resistance heat is produced and the loss of energy causes the voltage to decrease. This decrease is called the voltage drop.
In Figure 26.6, voltmeter number one (V1) is connected across the source; therefore, the reading on the voltmeter will be equal to the source, assuming no resistance in the conductors. Voltmeter number two (V2) is connected across the 3.2 ^ resistor. It will measure the difference in voltage from one side of the resistor to the other—in other words, the voltage drop across the resistor.
Problem: What will voltmeter number 2 read in Figure 26.6 if the source voltage, measured by voltmeter number 1 is 120 V?
Solution: Voltmeter number 2 is measuring the voltage drop caused by the 3.2 ^ resistor. Voltage drop is caused by current passing through a resistance, therefore to calculate the voltage drop across the resistor, the total current flow in the circuit must be known. The first step is to calculate the total current flow in the circuit.
FIGURE 26.6. Voltmeters in series circuit.
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