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
