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1-43 Q53. What is a multimeter? Q54. Why is a multimeter preferred over separate meters? Q55. How is a multimeter changed from a voltage measuring device to a resistance measuring device? Q56. Why is the dc scale on a multimeter different than the ac scale? Table 1-2 illustrates an interesting point about multimeters. It was mentioned earlier in this chapter that both voltmeters and ammeters have an effect upon the circuits they measure. CURRENT TO DEFLECT FULL SCALE METER MOVEMENT RESISTANCE VOLTMETER SENSITIVITY VOLTAGE FULL SCALE SHUNT RESISTOR OVERALL RESISTANCE   1mA                N      92/7 .1 V NA        \$              N      92/7    .048 V                \$          N     92/7   .029 V                       Table 1-2.—Multimeter Movements To keep this effect to a minimum, it is necessary that the voltmeter have a high resistance (sensitivity expressed in ohms per volt) and the ammeter have a low resistance compared to the circuit being measured. Table 1-2 shows the figures associated with three meter movements available for use in multimeters. The last two columns indicate the value of shunt resistance and the overall resistance of the shunt and meter movement necessary to compensate all three movements to an ammeter sensitivity (full-scale current) of 1 milliampere. Notice that as the voltmeter sensitivity increases, the resistance of the ammeter decreases. This shows how a meter movement used in a voltmeter will have a high effective resistance and the same meter movement used in an ammeter will have a low effective resistance because of the shunt resistors. PARALLAX ERROR Most multimeters (and some other meters) have a mirror built into the scale. Figure 1-39 shows the arrangement of the scale and mirror.