ELECTROMETER tik. .6XEKrpop, elek iron, amber farpov. Met run, in strument for measuring the difference of electro static potential between two charged conductors. The electroscope (q.v.) and the torsion balance q.v.) are forms of electrometers. and can be used to indicate the amount of electricity with which a given mduct or is charged : hut there are finer and more accurate instruments which are also based on the attraction and repulsion of two eharged conductors. In the Peltier electrometer a thin vane of aluminium has attached to it a magnet. whose moment is known, and which, being suspended at a point where the intensity of the earth's magnetism has been ascertained, enables us to determine the amount of any force which deflects it. There is a similar fixed arm which is charged with electricity of one kind. while the first vane is charged with elec tricity of the same kind. Consequently. there is a repulsive action between the two. whose ex tent depends upon the charges which the two conductors have. The capacity of these con ductors is always the same: consequently, the amount of electricity with which they are charged must depend upon the difference of potential; and this rule holds good in most forms of elec trometers. A more accurate form of electrometer consists in balancing the mutual attraction of two charged plate- by weights. A charged disk with a plain surface is attached to one arm of a balance, but eleetrieailv insulated from its metal lic parts. By means of balancing or the movement of the arm of the balance. the strength of attraction can he determined. This device, which was invented originally by now Harris. was later improved by Lord Kelvin, and was an instrument quite delicate in its action. In Lord Kelvin's it furnished valuable results, and was known as an absolute electrometer. lie al-o devised the quadrant electrometer. where tliere is a needle of thin aluminium suspended over four quadrants which are connected in pairs. A charge of electricity is communieated to one pair of quadrants diametrieally op posite each other, and a charge of the opposite kind is given to the other pair.
or they are connected with the earth. The needle.
therefore, will be attracted by one set of quadrants and repelled by the other, the amount of deflection measuring the potential diff•rence to which the quadrants are charged The action of the apparatus will be understood on reference to the illustration. Here the needle is charged by an electrophorus. or is in contact with a Leyden jar, and the two sets of are charged to the difference of potential which it is desired to measure. The amount of deflection can he determined accurately by using a reflecting mirror-on the needle and a telescope and scale, as is done with the galvanometer (q.v.), and it is
possible to measure even such small differences of potential as those of the two elements of it cell.
The form of electrometer illustrated is supplied with a replenisher in the shape of a small Leyden jar at its base, and the quadrants: are made double, or box shape. to increase the effect. An extremely useful example of the application of this principle is the electrostatic voltmeter used in the measuring of alternating currents. In this instrument. a vane of aluminium is deli cately pivoted, so that it may revolve between two brass plates from which it is insulated. The vane is connected with one eonducto• from the alternator. or other source of current. while the other conductor is connected with the fixed brass plates. When the vane and the plates are charged. there is a repulsive action between them, and the vane is revolved. The pointer attached to one arm moves over a graduated scale, indicat the difference of potential. and counter weights of different amounts may lie placed on the lower arm so that the range of the instrument can be varied and several sets of readings for different ranges of potential obtained. By in creasing the number of vanes and fixed plates, the sensitiveness of the instrument is increased. and it is then termed a multicellular voltmeter. An electrometer of quite different form is the capillary electrometer of Lippman and Dewar which is used to measure small differences of potential. This instrument consists of a glass tube placed horizontally, with its ends turned up and tilled with dilute acid. At the middle of this tulle is placed a drop of mercury. and as the current passes through the acid the drop travels along the tube toward the nega iVe pole. "Phis net ion is due to a ehange in the surface ten sion between the mercury and the acid, caused by the of gas. The volt meters already described are the ones that find the widest application and are used in industrial work. The others figure mostly in experiments in statical electricity and for accurate determina tions requiring careful manipulation. A descrip tion of electrometers wit be found in most of the works on electricity and physics, among which Thomps.in, Eionctitary in Electricity end Maya, f,srn 1 New York, 1901), can be recom mended as a simple treatise. For the• advanced student, Miiller-Pouillet-Plaundler. Lchrbuch (ler Physik (Brunswick. 1:+9S). will lie found to con tain a detailed description of the more important instruments. Willi(' the various journals dealing with physics can also lie referred to for further information.