Editing Electrometer (section)

=== Coulomb's electrometer ===
[[File:Coulomb electrometer.png|thumb|upright=0.4|Coulomb electrometer]]

Torsion is used to give a measurement more sensitive than repulsion of gold leaves or cork-balls. It consists of a glass cylinder with a glass tube on top. In the axes of the tube is a glass thread, the lower end of this holds a bar of gum lac, with a gilt pith ball at each extremity. Through another aperture on the cylinder, another gum lac rod with gilt balls may be introduced. This is called the carrier rod.

If the lower ball of the carrier rod is charged when it is entered into the aperture, this will repel one of the movable balls inside. An index and scale (not pictured) is attached to the top of the twistable glass rod. The number of degrees twisted to bring the balls back together is in exact proportion of the amount of charge of the ball of the carrier rod.

[[Francis Ronalds]], the inaugural Director of the [[Kew Observatory]], made important improvements to the Coulomb torsion balance around 1844 and the modified instrument was sold by London instrument-makers.<ref name="Ronalds 2016">{{Cite book|title=Sir Francis Ronalds: Father of the Electric Telegraph|last=Ronalds|first=B.F.|publisher=Imperial College Press|year=2016|isbn=978-1-78326-917-4|location=London}}</ref> Ronalds used a thin suspended needle rather than the gum lac bar and replaced the carrier rod with a fixed piece in the plane of the needle. Both were metal, as was the suspending line and its surrounding tube, so that the needle and the fixed piece could be charged directly through wire connections. Ronalds also employed a [[Faraday cage]] and trialled photography to record the readings continuously. It was the forerunner of Kelvin's quadrant electrometer (described below).