Ohmmeter: An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. A meter scale, resistor and a source are connected in series. Micro Ohmmeter, Mega Ohmmeter and Milli – Ohmmeters are used to measure resistance in different applications of electrical testing. Analog ohmmeter was used in earlier days while digital ohmmeter is used in the modern days.

Potential Divider: An instrument which divides the potential of a source is called potential divider. Rheostat can be used as a potential divider.

Potential divider

\begin{align*} \text {The current flowing, I in the circuit is given by  I} &= \frac {V}{R_1 + R_2} \\ \text {Then p.d. across,} \: R_1, V_1 = IR_1 = \frac {R_1}{R_1 + R_2} V \\ \text {The fraction of V obtained across} \: R_1 \text {is} \: \frac {R_1}{R_1 + R_2} \\ \text {If} \: R_1 = 10 \mho \text {and} \: R_2 = 90 \mho, \text {then} \: V_1 = \frac {10}{90 + 10} V = \frac {10 V}{100} = \frac {V}{10} \\ \end{align*}

A resistor with a sliding contact can be used as a potential divider which is shown in the figure above. This arrangement provides a continuously variable p.d. from zero to the full supply value.

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