is placed upon a charged body, the stem participates and repels the cork ball. The amount of repulsion may be read off the graduated semicircle, though the measured angle is not in direct proportion to the charge. Early inventors included William Henley (1770) and Horace-Bénédict de Saussure. Coulomb's 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. 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). Peltier electrometer Developed by Peltier, this uses a form of magnetic compass to measure deflection by balancing the electrostatic force with a magnetic needle. Bohnenberger electrometer The Bohnenberger electrometer, developed by J. G. F. von Bohnenberger from an invention by T. G. B. Behrens, consists of a single gold leaf suspended vertically between the anode and cathode of a dry pile. Any charge imparted to the gold leaf causes it to move toward one or the other pole; thus, the sign of the charge as well as its approximate magnitude may be gauged. Attraction electrometer Also known