Because 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 (RX = 0) so IM = IMFS. In this circuit, RH is still the
Thevenin resistance RX sees; this is