A voltmeter has an effect on the circuit being measured. This is called LOADING the circuit.

The figure below illustrates the loading effect and the way in which the loading effect is kept to a minimum.

The loading effect.

In view(A), a series circuit is shown with R1 equaling 15 ohms and R2 equaling 10 ohms. The voltage across R2 (ER2) equals 10 volts. If a meter (represented by R3) with a resistance of 10 ohms is connected in parallel with R2, as in view(B), the combined resistance of R2 and R3 (Rn) is equal to 5 ohms. The voltage across R2 and R3 is now 6.25 volts, and that is what the meter will indicate. Notice that the voltage across R1 and the circuit current have both increased. The addition of the meter (R3) has loaded the circuit.

In view(C), the low-resistance meter (R3) is replaced by a higher resistance meter (R 4) with a resistance of 10 kilohms. The combined resistance of R2 and R4 (Rn) is equal to 9.99 ohms. The voltage across R2 and R 4 is now 9.99 volts, the value that will be indicated on the meter. This is much closer to the voltage across R2, with no meter (R3 or R4) in the circuit.

Notice that the voltage
across R, and the circuit current in view(C) are much closer to the
values in view(A). The current (IR4) through the meter (R4) in view(C)
is also very small compared to the current (IR2) through R 2. In view(C)
the meter (R4) has much less effect on the circuit and does not load
the circuit as much. Therefore, a voltmeter should have a high
resistance compared to the circuit being measured, to minimize the
loading effect.

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 the figure below view(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 view(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.

Current and voltage in a parallel circuit.