MAKING A VOLTMETER FROM A CURRENT SENSITIVE METER MOVEMENT

1-27 Q30. What electrical quantity is measured by a voltmeter? Q31. How is a voltmeter connected to the circuit to be measured? Q32. What is the loading effect of a voltmeter? Q33. How is the loading effect of a voltmeter kept to a minimum? MAKING A VOLTMETER FROM A CURRENT SENSITIVE METER MOVEMENT The meter movements discussed earlier in this chapter have all reacted to current. Various ways have been shown in which these movements can be used in ammeters. If the current and resistance are known, the voltage can be calculated by the formula E = IR. A meter movement has a known resistance, so as the movement reacts to the current, the voltage can be indicated on the scale of the meter. In figure 1-27(A), a voltmeter (represented by R₂) connected across a 10-ohm resistor with 10 volts applied. The current through the voltmeter (R₂) is .1 milliamperes. In figure 1-27(B), the voltage is increased to 100 volts. Now, the current through the voltmeter (R₂) is 1 milliampere. The voltage has increased by a factor of 10 and so has the current. This illustrates that the current through the meter is proportional to the voltage being measured. Figure 1-27.—Current and voltage in parallel circuit. SENSITIVITY OF VOLTMETERS Voltmeter sensitivity is expressed in ohms per volt (W/V). It is the resistance of the voltmeter at the full-scale reading in volts. Since the voltmeter’s resistance does not change with the position of the pointer, the total resistance of the meter is the sensitivity multiplied by the full-scale voltage reading. The higher the sensitivity of a voltmeter, the higher the voltmeter’s resistance. Since high resistance voltmeters