ELECTROMETER

an electrical instrument for measuring differences of potential, by means of statical charges and not by currents. The form of instrument most frequently met with is the quadrant electrometer. In Lord Kelvin’s instrument of this type, a thin aluminum needle, dumb-bell in shape, is symmetrically suspended by one or two fibres within four flat hollow metal quadrants in such a way that its centre coincides with the centre of the quadrants. The opposite pairs of quadrants are connected by wires, and a charge is imparted to the needle, which is kept at constant potential by being connected with the inner coating of a Leyden jar arrangement in the base of the instrument. If the quadrant pairs are now charged to different potentials, the needle is attracted by one pair and repelled by the other pair, and thus the fibre is twisted by an amount proportional of the square of the potential difference. A small reflecting mirror attached to the fibre registers the amount of deflection on a scale in the usual manner. A very much simpler and more sensitive form of quadrant electrometer is the Dolezalek type. In this instrument the needle is very light, being of silvered paper, and the suspension is made by means of a fine quartz or metal fibre. For measuring small potential differences the needle is independently charged to a high potential. For large potential differences or rapidly alternating potentials the needle is attached to one pair of quadrants, High voltage voltmeters (electro-static voltmeters), utilizing this principle are now in use. Instruments called electro-static ground detectors working on the same principle are used to determine the condition of the ag of high voltage trans mi on es.