"The electrometer is the fundamental instrument for measuring potential difference (which is measured in volts), and depends on attraction or repulsion of charges on plates or wires. The measurement can be done by weighing, in which case the charge is recorded directly in terms of weight. Of the early electrometers, one of the most important was the torsion balance (130) of Charles Augustin Coulomb (1736-1806). This was used in 1785 for experiments to determine the distribution of electric charge over conductors of different shapes, and in experiments to verify the law of inverse square of the distance....Accuracy was introduced into such measurements by William Thomson, Lord Kelvin (1824-1907) in 1853, by means of insulation, drying the air inside the instrument, and ensuring careful screening from external interference" (Turner 1983,200). The earliest form of the quadrant electrometer (132,133) was also described by Belfast-born William Thomson in 1867. Thomson's initial goal was to provide an instrument which could continuously monitor the charge in a telegraphic cable (Lyall 1991,p.59). The term "electroscope" is used for a qualitative instrument, whereas the term "electrometer" is used for an instrument which gives a quantitative measurement.

130 Coulomb Torsion Balance
131 Henley Electrometer
132 Kelvin Quadrant Electrometer
133 Quadrant Electrometer

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