A device that can measure the resistance directly is known as an ohmmeter. There are two types of ohmmeters as
Series Types Ohmmeter
Shunt Type Ohmmeter
The circuit of a series type ohmmeter consists of a D’Arsonval galvanometer (PMMC meter) connected in series with a resistance and a battery. It has two terminals (A and B) to which the unknown resistance (Rx) is connected.
Since the meter and unknown are in series, the current through the meter depends upon the value of unknown resistor. The meter indication is directly proportional to the value of unknown resistor, provided that calibration problems are considered.
When Rx = 0 i.e. terminals A and B are short circuited, then maximum current flows in the circuit. Under this condition, the zero adjustor R2 is adjusted until the meter indicates full scale deflection current (Ifsd). This position of the pointer is marked 0 Ω on the scale.
When Rx = ∞ i.e. terminals A and B are open circuited, then no current flows in the circuit and the meter indicates zero current. This position of the pointer is marked ∞ Ω on the scale.
The intermediate markings can also be done on the scale by connecting different known values of Rx across the AB-terminals of the instrument.
Now, with the zero adjustor R2 being open, when an unknown resistor Rx across the AB-terminals is connected, a current Im will flow through the meter and is given by,
Since, the value of current Im is more than zero and less than full scale deflection current (Ifsd), therefore the meter will show a reading between zero and infinity. Thus, the value of unknown resistor Rx can be determined
The circuit of shunt type ohmmeter consists of a battery in series with a variable resistor R1 and a PMMC meter. An unknown resistor Rx is connected across the terminals A and B, in parallel with the meter. An on-off switch (S) is provided to disconnect the battery, when the instrument is not in use.
When Rx = 0, i.e., terminals A and B are short circuited, the meter current (Im) is zero, this position of pointer is marked 0 Ω on the scale.
When Rx = ∞ i.e. terminals A and B are open circuited, then the whole current will flow through the meter only and by adjusting the value of R1, the meter current (Im) can be set to full scale deflection current (Ifsd). The position of the pointer is, then marked ∞ Ω on the scale.
The shunt type ohm meter is mainly used for the measurement of low value resistances.