VoltmeterdesignAs was stated earlier, most meter movements are sensitive devices. SomeD'Arsonval movements have full-scale deflection current ratings as little as 50 µA,with an (internal) wire resistance of less than 1000 Ω. This makes for a voltmeterwith a full-scale rating of only 50 millivolts (50 µA X 1000 Ω)! In order to buildvoltmeters with practical (higher voltage) scales from such sensitive movements,we need to find some way to reduce the measured quantity of voltage down to alevel the movement can handle.Let's start our example problems with a D'Arsonval meter movement having afull-scale deflection rating of 1 mA and a coil resistance of 500 Ω:Using Ohm's Law (E=IR), we can determine how much voltage will drive thismeter movement directly to full scale:E = I RE = (1 mA)(500 Ω)E = 0.5 voltsIf all we wanted was a meter that could measure 1/2 of a volt, the bare metermovement we have here would suffice. But to measure greater levels of voltage,something more is needed. To get an effective voltmeter meter range in excess of1/2 volt, we'll need to design a circuit allowing only a precise proportion ofmeasured voltage to drop across the meter movement. This will extend the metermovement's range to being able to measure higher voltages than before.Correspondingly, we will need to re-label the scale on the meter face to indicateits new measurement range with this proportioning circuit connected.