1-26
Figure 1-26.—The loading effect.
In figure 1-26(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 figure 1-26(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 figure 1-26(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 figure 1-26(C) are much closer to the values in 1-26(A). The current (IR4) through the
meter (R4) in figure 1-26(C) is also very small compared to the current (IR2) through R
2. In figure 1-26(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.
