What do voltmeters measure?

Abstract

Purpose

The paper aims at justifying the operational rule “a voltmeter's reading is the electromotive force, as it existed before branching it, along the path γ traced out by the connectors between the contact points”.

Design/methodology/approach

A simple application of Faraday's law is enough to make the result plausible. Then it is shown that by an asymptotic analysis with the radius of the leads (assumed perfectly conductive) as small parameter that the current derived by the voltmeter is negligible.

Findings

The rule is valid in spite of the considerable modification of the electric field that branching a voltmeter entails.

Originality/value

“Voltage drop”, between A and B simply does not make sense. The threads of a voltmeter should carefully be placed in order to measure exactly what one has in view. This is explained by a few examples.

Citation

Bossavit, A. (2008), "What do voltmeters measure?", COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, Vol. 27 No. 1, pp. 9-16. https://doi.org/10.1108/03321640810836582