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 day- of incandescent lighting. In this instru ment the current is measured by the amount of chemical action which 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 a 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 0.000337, the electro•chemical equivalent of zinc, to obtain the amount of current in the branch circuit. and then multiplying by the ratio of the resistance of this branch circuit to the German silver re sistance across which it is connected. The ehemi Cal nu•er was found quite accurate. as it was possible to compensate for changes in the tem perature and make other corrections; but it possessed the di-advantage that it could not be read by the consumer. while the collection am] weighing of the plates wa- both troublesome and expensive. A form of electric nester in wide use i- a recording watt-meter. which is in its essentials electrie motor, designed by Prof. Elihu Thomson. It can be used for either direct or alternating current, and has dials similar to those of an ordinary gas meter, so that it may be read at any time not only by the in spector. hut also by the consumer. In this in strument the current from one of the main conductors through the coils of the field 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 thwed depends entirely upon the current passing through the field-magnets. and as the current in the armature is constant, being independent of all considerations save the difference 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 revolve, 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 au 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 registering 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, but dithering 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 electremag net, between the poles of whieh there is placed a shallow dish of mercury through which the cur rent also flows. (-Merin!" at the centre IT 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 of a vane which dips into the mercury. and suitable spindles 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 tion: must vary directly as the currents and a nil are recorded.
A meter in practice must be accurate to within 2 or 3 per cent. and be so simple in its construc tion and operation as to require little care, as it is the usual and commonplace method of ascer taining how much current is actually consumed by the patrons of a power station; and its impor tance is recognized and improvements are con stantly being made in both design and construc tion. In fact, the increased use of electric lighting has given rise to a form of meter known a- the 'prepayment meter,' where the insertion of a coin will so affect the mechanism that the appropriate amount of current is allowed to pass through the mains. For a description of electric meters of all types, the reader can consult Crocker. Electric Lighting, vol. ii. (New York, 1901).