THE VOLTMETER
Recall that one definition of voltage is electrical pressure. The voltmeter indicates the amount of potential between two points, in much the same way a pressure gauge indicates the pressure difference between two points. In the circuit shown in Figure 40–1, assume that a voltage of 120 volts exists between L1 and N. If the leads of a voltmeter were to be connected between L1 and N, the meter would indicate 120 volts. Now assume that the leads of the voltmeter are connected across the lamp, Figure 40–2.
QUESTION 1: Assuming the lamp filament is good, would the voltmeter shown in Figure 40–2 indicate 0 volts, 120 volts, or some voltage value between 0 and 120?
ANSWER: The voltmeter would indicate 0 volts. In the circuit shown in Figure 40–2 the switch and lamp are connected in series. One of the basic rules for a series circuit is that the voltage drop across all components equals the applied volt- age. The voltage drop across each component is proportional to the amount of resistance of the component and the amount of current flow. In the circuit shown in Figure 40-2, there is no cur- rent flow because the switch is open. Because no current can flow through the lamp there can be no voltage drop.
QUESTION 2: If the voltmeter probes were to be moved across the switch as shown in Figure 40–3 would the meter indicate 0 volts, 120 volts, or some value between 0 and 120 volts?
ANSWER: The voltmeter would indicate 120 volts. Because the switch is an open circuit, the resistance is infinite at this point, which is millions of times greater than the resistance of the lamp filament. Remember that voltage is electrical pressure. The only current flow necessary to measure voltage is the current flow through the meter itself. In this circuit, the only current path is through the resistance of the voltmeter and the lamp filament, Figure 40–4. If the probes of the voltmeter were to be connected to a wall outlet the meter would indicate 120 volts, but there would be no current flow except through the meter itself.
QUESTION 3: If the total or applied voltage in a series circuit equals the voltage drop across each component, why is all the voltage drop across the voltmeter resistor and none across the lamp filament?
ANSWER: There is some voltage drop across the lamp filament because the current of the voltmeter is flowing through it. The voltage drop across the lamp filament, however, is so small as compared with the voltage drop across the voltmeter resistance it is generally considered to be zero. Assume the lamp filament to have a resistance of 50 ohms. Now assume the voltmeter is a digital voltmeter with a resistance of 10 million ohms. The total circuit resistance is 10,000,050 n. The total circuit cur- rent is 0.000011999 amps (120/10,000,050) or about 12 microamperes. The voltage drop across the lamp filament is 0.0006 volts or 0.6 millivolts (50 X 12 μA).
QUESTION 4: Assume that the filament of the lamp is open or burned out. Would the voltmeter in Figure 40–3 indicate 0 volts, 120 volts, or some value between 0 and 120 volts?
ANSWER: The voltmeter would indicate 0 volts. If the lamp filament is open or burned out, a current path for the voltmeter does not exist and the voltmeter would indicate zero, Figure 40–5. In order for the voltmeter to indicate voltage, it would have to be connected across L1 and N so that a complete circuit through the meter would exist, Figure 40–6.
