# An ohmmeter works

﻿Keywords: an ohmmeter works
Description: There are a bunch of different approaches that at have been used. For a long time the most common type was a combination of a battery, a moving-coil current meter, and two resistors, one fixed and

There are a bunch of different approaches that at have been used. For a long time the most common type was a combination of a battery, a moving-coil current meter, and two resistors, one fixed and one adjustable. See OHMMETERS. It was a pretty kludgy method, but it was cheap and adequate enough for many purposes.

Summarizing, the battery voltage would be used to drive a current through the resistance to be measured, the coil, and both internal resistors, all in series. The internal resistors would be chosen to give full scale current on the meter when the test terminals were shorted, i.e. a test resistance of zero. For example, if a 4.5 V battery and a 1 mA meter were used (as at the above link), the internal resistances would have to add up to 4.5 kohm.

The rest was done by painting a fancy nonlinear, backwards scale on the face of the meter. On the left, the zero current position would be labelled $\infty$. i.e. infinite resistance. On the right, the full current position would be labelled 0 (i.e. the internal resistance plus zero extra). In the middle, the halfway point would be labelled to match the total internal resistance (after all, if you have a certain internal resistance and then connect the same amount of resistance externally you'll get half the full current).

The variable resistor was used to compensate for variations in the battery voltage. As a preliminary step you would need to short the leads and tweak the adjustment knob until you really did get full scale.

A realistic ohmmeter of this type would usually have a selector knob to switch in different combinations of battery voltage and internal resistors so as to cover different current ranges.

Of course now that we have dirt-cheap amplifier circuits, the old approach is obsolete. Digital multimeters tend to use a feedback circuit to create a fixed current source, and then measure the voltage that that current produces.