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METER, ELECTRIC
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See also:METER, ELECTRIC . In the public See also:supply of electric See also:energy for See also:lighting and See also:power it is necessary to provide for the measurement of the electric energy or quantity by devices which are called electric meters. Those in use may be classified in several ways: (i) according to the See also:kind of electric supply they are fitted to measure, e.g. whether continuous current or alternating current, and if the latter, whether monophase or polyphase; (ii) according to whether they See also:record intermittently or continuously; (iii) according to the principle of their See also:action, whether See also:mechanical or electrolytic; (iv) according to the nature of the measurement, whether quantity or energy meters. The last subdivision is fundamental. Meters intended to measure electric energy (which is really the subject of the See also:sale and See also:purchase) are called See also:joule meters, or generally See also:watt-See also:hour meters. Meters intended to measure electric quantity are called See also:coulomb meters and also See also:ampere-hour meters; they are employed for the measurement of public electric supply on the See also:assumption that the electromotive force or pressure is See also:constant. Most of the See also:practical meters in use at the See also:present See also:time may be classified under the following five heads: electrolytic meters, motor meters, See also:clock meters, intermittent registering meters and See also:induction meters. Electrolytic Meters are exclusively ampere-hour meters, measuring electric quantity directly and electric energy only indirectly, on the assumption that the pressure of the supply is constant. The first electrolytic See also:house meter in connexion with public electric supply was described by St. See also:George See also:Lane-See also:Fox. He was followed by F. J. Sprague and T. A. See also:Edison, the last-named inventor elaborating a type of meter which he employed in connexion with his See also:system of electric lighting in its See also:early days. The Edison electric meter, like those of Sprague and Lane-Fox, was based upon the principle that when an electric current flows through an electrolyte, such as sulphate of See also:copper or sulphate of See also:zinc, the electrodes being plates of copper or zinc, See also:metal is dissolved off one See also:plate (the anode) and deposited on the other plate (the See also:cathode). It consisted of a See also:glass See also:vessel, containing a See also:solution of sulphate of zinc, in which were placed two plates of pure amalgamated zinc. These plates were connected by means of a See also:german-See also:silver shunt, their See also:size and the distance between them being so adjusted that about iaisa See also:part of the current passing through the meter travelled through the electrolytic See also:cell and of the current passed through the shunt. Before being placed in the cells the zinc plates were weighed. The shunted voltameter was then inserted in See also:series with the electric supply mains leading to the house or See also:building taking electric energy, and the cur-See also:rent which passed dissolved the zinc from one plate and depos d it upon the other, so that after a certain See also:interval of time had elapse the. altered See also:weight of the plates enabled the quantity of See also:electricity to be determined from the known fact that an electric current of . • one ampere, flowing for one hour, removes 1.2133 grammes of zinc from a solution of sulphate of zinc. Hence the quantity in ampere-See also:hours passing through the electrolytic cell being known and the fraction of the whole quantity taken by the cell being known, the quantity supplied to the house was determined. To prevent temperature from affecting the shunt ratio, Edison joined in series with the electrolytic cell a copper coil the resistance of which increased with a use of temperature by the same amount that the electrolyte decreased. Owing to the cost and trouble of weighing a large number of zinc plates, this type of meter See also:fell into disuse. done at the magnificent astronomical observatories that are so numerous throughout the See also:world. Every atmospheric phenomenon can be materially elucidated by exact laboratory experiments and measurements. theory can be confronted with facts; and the student can become an See also:original investigator in See also:meteorology. The See also:great difficulties inherent to meteorology should stimulate the devotion of the highest See also:talent to the progress of this See also:branch of See also:science. The practical value of See also:weather predictions justifies the See also:expenditure of See also:money and labour in See also:order to improve them A more See also:modern type of electrolytic meter is that due to C. O. See also:Bastian.' The whole current supplied to the house flows through an electrolytic cell consisting of a glass See also:tube containing two See also:platinum electrodes; the electrolyte Is dilute sulphuric See also:acid covered with a thin layer of oil to prevent evaporation. As the' current flows it decomposes the liquid and liberates See also:oxygen and See also:hydrogen gases, which See also:escape. The quantity of electricity which is passed is estimated by the diminution in the See also:volume of the liquid. A third electrolytic meter of the shunted voltameter type is that of A. See also:Wright. In this meter the electrolyte is a solution of mercurous nitrate which is completely enclosed in a glass tube of a particular See also:form, having a See also:mercury anode and a platinum or, See also:carbon cathode. The current is determined by measuring the volume of the mercury delivered at the cathode. In the See also:Long-Schattner electrolytic meter a solution of sulphate of copper is electrolyzed.,
Motor Meters.—Amongst 'motor meters one well-known type be-longing to the ampere-hour See also:species is that of S. Z. Ferranti, who introduced it in 1883. It consists of an electromagnet within the See also:iron core of which is a See also:flat disk-like cavity containing mercury, the sides of the cavity being stamped with grooves. The thin disk of mercury is therefore traversed perpendicularly by lines of magnetic force when the magnet is excited. The current to be measured is passed through the coils of the electromagnet, then enters the mercury disk at the centre, flows through it radially' in all directions, and emerges at the periphery. The See also:mass of mercury is thus set in See also:motion owing to the tendency of a conductor conveying an electric current to move transversely across lines of magnetic force; it becomes in fact the See also:armature of a See also:simple form of, See also:dynamo, and rotates with a See also:speed which increases with the strength of the current. The roughness of the See also:surface of the cavity serves to retard it. The rotation of the mercury is detected and measured by means of a small See also:vane of platinum See also:wire immersed in it, the See also:shaft of this vane being connected by an endless See also:screw with a counting mechanism. The core of tke electromagnet is worked at a point far below magnetic saturation (see See also:MAGNETISM) ; hence the See also: Another well-known motor meter, working on a somewhat similar principle, is that pf See also: The current to be measured passes through the fixed field-coils, whilst through the armature passes a, shunt current obtained by connecting the brushes across the supply mains through a constant resistance. The driving force is balanced against a retarding force produced by the rotation of a copper disk fixed on the armature shaft, which rotates between the poles of a permanent magnet. Induced or eddy currents are thus created in the copper disk, and the reaction of these against the magnetic field offers a resistance to the rotation of the disk. Hence when a current is passed through the meter, the armature rotates and increases its speed until the driving force is balanced against the retarding force due to the eddy currents in the copper brake disk. In these circumstances the number of rotations made by the armature in a given time is proportional to the product of the strength of the ' current flowing through the armature and that flowing through the field-coils, the former being the current to be measured. Hence the meter is a watt-hour meter and See also:measures electric energy. In order to overcome the friction of the train the field-coils are See also:wound with an See also:auxiliary shunt coil which supplies a driving force sufficient to over-come the friction of the counting train. This last is geared to the shaft of the armature by an endless screw, and the number of revolutions of the armature is reckoned by the counting-dials, which are See Electrician, 41, 112, and Journ. Inst. Elec. Eng. (See also:London, 1898), 27, 547.so arranged as to indicate the See also:consumption in See also:Board-of-See also:Trade See also:units (1 Board-of-Trade unit =t000 watt-hours). A modification of the above meter with some mechanical improvements has been devised by S. Evershed? Clock Meters.--Among clock meters the best known is ' that of H. Aron, which is based upon a principle described by W. E. See also:Ayrton and J: See also:Perry in 1882. It can be constructed to be either an ampere hour meter or a watt-hour meter, but is usually the latter. Its principle is as follows: Suppose there are two pendulum clocks, one having an See also:ordinary pendulum and the other having a pendulum consisting of a See also:fine coil of wire through which a current is• passed. proportional to the potential difference of the supply mains—in other words, a shunt current. Below this pendulum let there be placed another coil through which passes the current to be measured then when' currents pass through these coils the pendulum of the second clock will be either accelerated or retarded relatively: to the other clock, since the action of gravity is supplemented by that of an electric attraction or repulsion between the coils. Hence the second clock will gain or lose on the other. The two clock motions may be geared to a single counting mechanism which records the difference in the rates of going of the two clocks. If the difference of the number of oscillations made by the two pendulums in a given time is small compared to the number made by either of them separately, then it is easy to show that the power given to the See also:circuit is measured by the gain or 'loss of one clock over the other in a given time, and can therefore be indicated on a counting mechanism or registering dials. By the use of a permanent magnet instead of a shunt coil as the bob of one pendulum, the meter can be made up as an ampere-hour meter. In this form it has the See also:advantage that it can be used for either continuous or alternating currents. In Intermittent Registering Meters some form of ampere-meter or watt-meter registers the current or power, passing into the house; and a clock motion electrically driven is made to take readings of the ampere-meter or watt-meter at definite intervals—say, every five minutes—and to add up these readings upon a set of registered dials. The arrangement therefore integrates the ampere-hours or watt-hours. These meters, of which one well-known form is that of ohnson and See also:Phillips, have the disadvantage of being unsuited for the measurement of electric supply in those cases in which it is irregular or intermittent—as in a See also:theatre or hotel. Induction Meters are applicable only in the See also:case of alternating current supply. One of the most widely used forms is the Westing house-Shallenberger. It consists of a disk of See also:aluminium, the See also:axis of which is geared to a counting mechanism and which runs between the poles of permanent magnets that create eddy currents in it and therefore exert a retarding force. In proximity to the upper See also:side of the disk is placed a coil of wire having an iron core, 'which is a shunt coil, the ends of the coil being connected to the terminals of the supply mains. Under the disk are two other coils which are placed in series with the supply. When these last coils are traversed by an alternating current they induce local or eddy currents in the disk. The current inthe shunt coil lags 90 degrees behind the impressed electromotive force of the circuit to be measured; hence if the See also:main current is in step with the potential difference of the terminals of the supply mains, which is the case when the supply is given wholly to electric lamps, then the field due to the main coil differs from that due to the shunt coil by, 90 degrees. Since the eddy currents induced in the disk are 90 degrees in phase behind the inducing field, the eddy currents produced by the main coil are in step with the magnetic field due to the shunt coil, and hence the disk rs driven See also:round by the revolution due to the action of the shunt coil upon the induced currents in the disk. Hence the disk will be accelerated until the driving force is balanced by the retarding force due to the induced currents created in the disk by the permanent magnets. When this is the case, the number of revolutions of the meter in a given time is a measure of the watt-hours or energy which is passed through the meter. The counting mechanism and dials may be so. arranged as to indicate this energy directly in watt-hours. The meter is made up also in a form suitable for use with two or three fixed electric currents. (See ELECTROIZINETICS.) Requirements of a See also:good House Meter.—A See also:gas meter which has an See also:error of more than 2% in favour of the seller or 3% in favour of the customer is not passed for use. An electricity meter should therefore have approximately the same accuracy. As a See also:matter of fact, it is difficult to rely upon most electric meters to See also:register correctly to less than 4% even between See also:quarter-load and full load. Out of nearly 700 current motor meters of various makes tested at See also:Munich in 1902, only 319 had an error of less than 4%, whilst 259 had errors varying from 4j to 1o%. If possible, how-ever, the departures from See also:absolute accuracy should not be more than 2% at quarter-load, nor more than 3% at a full load. The accuracy of a meter is tested by See also:drawing See also:calibration curves showing the percentage departure from absolute accuracy in, its See also:reading for various decimal factions of full load. Such a test is' made by determining with an accurate ammeter or watt-meter the current or power supplied to a circuit for a See also:period measured by a good: clock and comparing with this the actual reading of the meter 2 See Journ. Inst. Elec. Eng. See also:bond. (1899), 29, 743. during the same time. A See also:common source of trouble is the See also:short circuiting of the shunt coils owing to the shellaced See also:cotton covering of the wire becoming moist. A good meter should start with a current which is not more than 2% of its full load current. With a supply pressure of 200 volts a 5 c.p. carbon filament See also:lamp takes only o 1 ampere; hence unless a meter will begin to register with ampere it will fail to record the current consumed by a single small incandescent lamp. In a large supply system such failure would mean a serious loss of See also:revenue. The resistance of the meter coils causes a fall in voltage down the series coil which reduces the supply pressure to the consumer. On the other See also:hand the resistance of the shunt coil absorbs energy which generally varies from I to 3 See also:watts and is a loss either to the consumer or to the supply See also:company, according to the manner in which the shunt coil is connected. In those meters which are compounded—that is, have a shunt coil wound on the field magnets to compensate for the friction of the train—it is important to See also:notice whether the meter will operate or continue operating when there is no current in the series coil, since a meter which " runs on the shunt " runs up a See also:debt against the consumer for which it gives no corresponding advantage. Generally speaking, the See also:price of the meter is a subordinate See also:consideration. Since the revenue-earning power of a supply station depends entirely upon its meters, inaccuracy in meter record is a serious matter. The cost of measuring current by the aid of a meter is made up of three parts: (I) the See also:prime cost of the meter, which varies from £z to £6 for an ordinary 25-See also:light house electric meter; (2) the See also:capital value of the energy absorbed in it, which if the cost of the energy is taken at 2d. per Board-of-Trade unit, with See also:interest and depreciation at 6 %, may amount to £to per customer; and (3) the See also:annual working See also:costs for See also:repairs and also the See also:wages of the See also:staff of meter men, who take the required monthly or quarterly readings. In the case of small and irregular consumers, such as the inhabitants of See also:model dwellings and flats inhabited chiefly by working-class tenants, See also:coin-in-the-slot meters are much employed. The customer cannot obtain current for electric lighting until he has placed in a slit a certain coin—say, a See also:shilling—entitling him to a certain number of Board-of-Trade units—say, to 2 or 4, as the case may be. In the Long-Schattner electrolytic meter, the insertion of the coin depresses a copper plate or plates into an electrolytic cell containing a solution of sulphate of copper; the passage of the current dissolves the copper off one of the plates, the loss in weight being determined by the quantity of the electricity passed. As soon as the plate has lost a certain amount of weight corresponding to the value of the electric energy represented by the coin, the plate rises out of the liquid and cuts off the current. Additional information and CommentsThere are no comments yet for this article.
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