ELECTRIC LIGHTING. rent is employed, with step-down lr:iiisforiners to reduce the potential, or rotary i-onverters to clianjre it to direct current. Ai;;iin>t tlie advan- tages due to the simple generating maoliinery (see Dy.x.vmo - Electkic M.vciunkhy). and the smaller wires used in the alternating system, there is the cost of transformers and extra pre- cautions and dangers incident to high-potential current. The accom])anying table shows the rela- tive number of lamps under each system early in 1S99, and also the relative number of arc and incandescent lamps in service at the same time. Incandescent lamps are invariably used for do- mestic and most office lighting, and. in general, where a thoroughly distributed light which is not glaring in its brilliancy at any point is de- sired. Arc lamps are more commonly employed than incandescent lamps for street lighting, and are much u*ed in stores and some classes of large assembly halls or rooms. The inclosed are lamp, the use of arcs of lesser candle power, and the combination of arc and incandescent lamps on the same circuit, permit a wide range of choice to suit local needs or preferences. Bibliography. Crocker, KIcclric Lighting (Xew York, 1901). an excellent and complete modern treatise: Houston and Kenelly, Electric Arc Lighting (Xew York, 189.5) ; id.. Electric Incandescent Lighting (Xew York, 1895) ; Salo- mons. Electric Light Installations (3 vols., Lon- don. 1S94-98) : Slingo and Brooker, Electric Engi- neering for Electric Light Artisans and Students (London, 1898). See Dt>'AMO-Electric JIa- CHIXERY" for a description of dynamo and motors; Storage B.tterie.s for accumulators and storage systems: Transformers; Transmission of Power: Electric Meter. ELECTRIC METER. An instrument to measure and record the consumption of electrical energy. The first electric meter was the Edison chemical meter, used extensively in the early days of incandescent lighting. In this instru- ment the current is measured by the amount of chemical action which takes place in an electro- lytic cell. According to the law of Faraday, the amount of metal deposited on one electrode or dissolved from the other is proportional to the current passing between them. Use is made therefore of two electrodes of zinc, which are im- mersed in ,"1 zinc sulphate solution, and arranged as a shunt or branch circuit across a given re- sistance of German silver inserted in the main circuit. The plates are collected each month and accurately weighed, and the consumption of cur- rent ascertained by dividing the loss in weight experienced by one of the plates by COOOS."!?. the electro-chemical equivalent of zinc, to obtain the amount of current in the branch circuit, and then multiplying by the ratio of tlie resistance of this branch circuit to the German silver re- sistance across which it is connected. The chemi- cal meter was found quite accurate, as it was possible to compensate for changes in the tem- perature and make other corrections: but it possessed the disadvantage that it could not be read by the consumer, while the collection and weighing of the plates was both troublesome and expensive. A form of electric meter in wide u*e i« a recording watt-meter, which is in its essentials a small electric motor, desigtied by Prof. Elihu Thomson. It can be used for either direct or alternating current, and has dials 771 ELECTRIC METEH. similar to those of an ordinary gas-meter, so that it may be read at any time not only by the in- spector, but also by the consumer. In this in- strument the current from one of the main conductors passes through the coils of the licld- magnets of the motor, while the armature circuit, which is connected with a resistance coil, is di- rectly across the mains as a shunt. There is a small commutator of silver, but the armature itself contains no iron. The magnetic field pro- duced depends entirely upon the current i)assing through the field-magnets, and as the current in the armature is constant, being independent of all considerations save the dilference of potential maintained across the mains, consequently the force tending to rotate the armature is equiva- lent to the product of the current in the field- coils, or the amount being measured, and that flowing through the armature. As a brake on the motion of the armature there is a disk of copper mounted on its shaft, which revolves be- tween the poles of a number of permanent mag- nets, with the result that Foucault currents are produced which tend to reduce the speed of revo- lution. Another direct reading meter based on the- motor principle is the Shallenberger meter, used exclusively to measure alternating currents. By using a closed copper coil inside of. and at an angle to. a main coil through which the current passes, a rotating field is produced by the alter- nating current, which causes a thin disk of metal, fastened to a vertical axis and connecting with the regist<?ring mechanism, to revolve. This spin- dle carries below the disk vanes of aluminium, which tend to retard its speed of rotation and makes the number of revolutions strictly propor- tional to the current. While the Shallenberger meter records merely current-hours, another in- strument of similar form, known as the Westing- house integrating meter, is provided with a shunt winding where the effect produced depends upon the voltage. This enables watt-hours to be re- corded. There are also other instruments based on these same general properties, bvit differing in details of design. In other forms of meter, clock- work or pendulums are utilized, and an electro- magnet, through whose coils the current passes, affects the mechanism so that a record is made of the passing current. The principle that a con- ductor over which passes a current when placed in a magnetic field will seek to place itself perpendicular to the current and the lines of force has been utilized in the de- sign of the Ferranti meter which is employed in English practice. In this instrument the main current passes through an electromag- net, between the poles of which there is ])laced a shallow dish of mercury through which the cur- rent also flows, entering at the centre by a metal- lic pin and passing out through the surrounding rim. The mercury is set into revolution in the passage of the current, and this motion is com- municated to a system of dials by means of a vane which dips into the mercun-. and suitable spindlfs and gears. The chamber containing the mercury is so grooved that the friction produced, which is the force that the revolving mercury has to overcome, is proportional to the square of the speed; and, as the driving force varies with the square of the current, the number of revolu- tions must vary directly as the currents and ampere-hours are recorded.
