3-16center-zero- (0 reading is in the center) indicating instrument. Observing the correct polarity is importantwhen connecting the instrument to the circuit. Voltmeter polarity is the same as for the dc ammeter; thatis, current flows from negative to positive.Q-17.In what manner are voltage-measuring instruments connected to the circuit to be measured?Influence of a Voltmeter in a CircuitThe purpose of a voltmeter is to indicate the potential difference between two points in a circuit.When a voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has a low resistance,it will draw a substantial amount of current. This action lowers the effective resistance of the circuit andchanges the voltage reading. When you are making voltage measurements in high-resistance circuits, usea HIGH-RESISTANCE VOLTMETER to prevent the shunting action of the voltmeter. The effect is lessnoticeable in low-resistance circuits because the shunting effect is less. The problem of voltmetershunting (sometimes called circuit loading) is illustrated in figure 3-11.Figure 3-11.—Shunting action caused by a voltmeter.Q-18.When making voltage measurements in a high-resistance circuit, you should always use avoltmeter with what relative value of resistance?In view A of figure 3-11, a source of 150 volts is applied to a series circuit consisting of two 10-kilohm resistors. View A shows the voltage drop across each resistor to be 75 volts. In the 150-volt range,the voltmeter to be used has a total internal resistance of 10 kilohms. View B shows the voltmeterconnected across the circuit. The parallel combination of R2 and the meter now present a total resistanceof 5 kilohms. Because of the addition of the voltmeter, the voltage drops change to 100 volts across R1and 50 volts across R2. Notice that this is not the normal voltage drop across R2. Actual circuit conditionshave been altered because of the voltmeter.Voltmeter SensitivityThe sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of theresistance of the meter (Rmeter), plus the series resistance (Rseries), by the full-scale reading in volts. Inequation form, sensitivity is expressed as follows:This is the same as saying the sensitivity is equal to the reciprocal of the full-scale deflection current.In equation form, this is expressed as follows:
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