3-14Since the normal voltage drop for the meter is 10 millivolts at full-scale deflection, some means mustbe supplied to drop the extra 9.99 volts without applying it directly to the meter. This is done by theaddition of a multiplier resistor, as shown in figure 3-9.Extending Voltmeter RangesThe value of series resistance is determined by the current required for full-scale deflection and bythe range of the voltages to be measured. Since the current through the meter circuit is directlyproportional to the applied voltage, the meter scale can be calibrated directly in volts for a fixed value ofseries resistance. For example, let’s assume that the basic meter is to be made into a voltmeter with a full-scale deflection of 1 volt. The coil resistance of the basic meter is 100 ohms, and 100 microamperes ofcurrent causes full-scale deflection. The resistance (R_{meter}) required to limit the total current in the circuitto 100 microamperes can be found as follows:Because the meter coil already measures 100 ohms, the series resistance required is equal to 10kilohms minus 100 ohms, or 9.9 kilohms.Q-16.What factors determine the value of the multiplier resistor?Multirange voltmeters use one meter movement. The required resistances are connected in serieswith the meter by a switching arrangement. A schematic diagram of a multirange voltmeter with threeranges is shown in figure 3-10. The total meter resistance (R_{meter}) for each of the three ranges, beginningwith the 1-volt range, is figured by the application of Ohm’s law, as follows:

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