Since the voltage, VB, of the series ohmmeter’s battery drops as the battery is used and ages, a variable resistor, RV, is added in parallel with the microammeter to compensate for the drop in VB. This practical series ohmmeter circuit is shown in Figure 8.64. RV is adjusted with the meter leads shorted (RX¼0), so IM¼IMFS. In this circuit, RH is still the Thevenin resistance that RX sees, which is: