Meter Resistance: Don't Forget It!

Abstract

When measuring potential difference or current with an analog meter employing a moving-coil galvanometer, the user has to be sensitive to circuit changes produced by the internal resistance of the meter. Fortunately, analog meters have been largely replaced with digital instruments that ordinarily have negligible effect on the measurements. Nevertheless, there are situations where a digital meter can seriously affect the circuit. We report here such a situation that was embarrassing initially but proved to be instructional in the end.