Henley's electrometer, c 1770.
© Science Museum / Science & Society Picture Library
A quadrant electrometer designed by William Henley and made by Adams. An electrometer is an instrument for measuring electrical potential without drawing any current from the circuit under test. Henley's quadrant electrometer of 1770 consisted of a semicircular scale divided in degrees and secured to a wooden or bras stem. The divergence was indicated by a light wooden rod terminating in a small pith or cork ball, and pivoted at the centre of the scale.