ELECTROLYSIS OF MAINS. 783 ELECTROMETER. either tho ilouble overheailtrolley system or an uiiderjirouiul return wire slioiilil be provided. Consult the Journal of tlw Ww England )1'«((;- H'orAs Axsociiition. and the Proi-cvdings of the Anicriciin Walcr-Worl:^ Association for profes- sional and tetliiiital p;i|HMs on this subject. ELECTRO-MAGNETIC INDUCTION. See 1>'UVCTH1N. ELECTRO - METALLURGY AND ELEC- TRIC SMELTING. S,r Mi i vi.i.i m:Y ; An Mi.- 11 M ; (niTti;: I :oLi> ; SlLKK: I'L.vn.MM ; ZlXC. ELECTROMETER (from Gk. iiXexTpov, elek- iron, anil)er -*- jiirpof. inctroii, measure I. An iu- strument for measuring the dilferenee of electro- static potential between two charged conductors. The electroscope (q.v. ) and the torsion balance (q.v. ) are forms of electrometers, and lau be used to indicate the amount of electricity with which a given ciiniluctor is charged; but there are finer and more accurate instruments which are also based on the attraction and i-epulsion of two charged conductors. In tlic Peltier electrometer a thin vane of aluminium has attached to it a small magnet, whose magnetic 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. Therc 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 difl'erence 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 plates by weights. A charged disk with a plain surface is attached to one arm of a balance, but electrically insulated from its metal- lic parts. By means of balancing weights or the movement of the arm of the balance, the strength of attraction can be determined. This device, which was invented originally by Snow Harris, was later improved by Lord Kelvin, and was an instrument qviite delicate in its action. In Lord Kelvin's hands it furnished valuable results, and was known as an al)solute electrometer. He also devised the quadrant electrometer, where titere is a needle of thin aluminium suspended over four quadrants which are connected in pairs. A charge of electricity is coninnuiicated to one pair of quadrants diametrically 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- dilTcrence to which the quadrants are charged The action of the apparatus will be imderstood 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 quadrants are charged to the diflerence of potential which it 13 desired to measure. The amount of deflection can be 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 jKissible til measure even such small dilTercnces of potential as those of the two elements of a cell. QUADBAXT ELECTROME- TER. THOMSOX ELECTROMETER. 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 mada 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 conductor from the alternator, or other source of current, while tho other condtictor is connected with the fixed brass ])lates. 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- ing the difference of potential, and counter- weights of different amounts may be placed on the lower arm so that the range of the instrument can he varied and several sets of readings for dilTerent 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 dillercnt form is the capillary electrometer of Lip])man and Dewar which is used to measure small difTcrences of potential. This instrument consists of a glass tube placed hcuizontally. with its ends turned up and filled with dilute acid. At the middle of this tube is placed a drop of mercury, and as the current passes through the acid the drop travels along the tube toward the negative pole. This action is due to a change in the surface ten- sion between the mercury jind the acid, caused by the liberation of gas. The electrostatic volt- meters already described are the ones that find the widest application and are used in industrial
