1-26Figure 1-26.—The loading effect.In figure 1-26(A), a series circuit is shown with R_{1} equaling 15 ohms and R_{2} equaling 10 ohms. Thevoltage across R_{2} (E_{R2}) equals 10 volts. If a meter (represented by R_{3}) with a resistance of 10 ohms isconnected in parallel with R_{2}, as in figure 1-26(B), the combined resistance of R_{2} and R_{3} (R_{n}) is equal to 5ohms. The voltage across R_{2} and R_{3} is now 6.25 volts, and that is what the meter will indicate. Notice thatthe voltage across R_{1} and the circuit current have both increased. The addition of the meter (R_{3}) hasloaded the circuit.In figure 1-26(C), the low-resistance meter (R_{3}) is replaced by a higher resistance meter (R_{4}) with aresistance of 10 kilohms. The combined resistance of R_{2} and R_{4} (R_{n}) is equal to 9.99 ohms. The voltageacross R_{2} and R4is now 9.99 volts, the value that will be indicated on the meter. This is much closer tothe voltage across R_{2}, with no meter (R_{3} or R_{4}) in the circuit. Notice that the voltage across R, and thecircuit current in figure 1-26(C) are much closer to the values in 1-26(A). The current (I_{R4}) through themeter (R_{4}) in figure 1-26(C) is also very small compared to the current (I_{R2}) through R_{2}. In figure 1-26(C)the meter (R_{4}) has much less effect on the circuit and does not load the circuit as much. Therefore, avoltmetershouldhaveahighresistancecomparedtothecircuitbeingmeasured,tominimizetheloadingeffect.

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