where V is the voltmeter reading, IX is the ammeter reading (current through RX) and
RAM is the resistance of the ammeter. The first method is best for measuring low values
of RX where RVM44RX and the second method has less error for RAM55RX. Obviously,
RX must be found by calculation and the accuracy of the result depends on the accuracy
of the meters.
The voltmeter-ammeter method of measuring resistance can be used to measure very
low resistances, of the order of m. An electronic current source is used to pass a known
amount of dc current through the unknown, low resistance. An electronic, dc
nanovoltmeter is connected across RX. Obviously, VM is directly proportional to RX.
Common, passive, benchtop volt-ohm-milliammeters (multimeters) use a simple series
ohmmeter circuit, shown in its simplest form in Figure 8.63, have a nonlinear (hyperbolic)
scale on the D’Arsonval microammeter with zero ohms full scale and infinite ohms
at zero deflection. A typical benchtop VOM series ohmmeter will have five RH ranges
(RH is the indicated RX at half-scale meter deflection), typically ranging 12 –120 k.
Inspection of the simple circuit shows that the dc meter current is given by