The Deutsch-Russische Battery Meter. The battery meter of the DeutschRussische Elektricitätszähler-Gesellschaft is illustrated in Fig. 86, and is a combination of two of their reverser armature systems operated upon by one main current coil. Each armature system is complete in itself, and is provided with an independent magnetic brake and counter, the one registering the charge and the other the discharge. One only of the two systems is influenced at any one time by the passage of a current in the series coil, and no mechanical

coupling between them is required. This follows at once from the principle of the meter, described on page 83.

The magnetic field produced by the main current causes the one reverser armature to pulsate during the charge and to register, holding back the other, which is oppositely polarised. When the current is reversed, the first armature will be prevented from moving, and the other will now be made to pulsate and register the discharge.

The O'K. Battery Meter. The Compagnie pour la Fabrication des Compteurs, Paris, slightly modify their O'K. ampere-hour meters when intended to measure the charge and discharge of a battery of accumulators.

Only one meter is used. The small electric motor of this meter, as already explained on page 43 in Chapter III., is connected as a shunt to a resistance which carries the whole current to be measured, and which is mounted on the base-plate of the meter itself. In this case, however, the resistance forms a separate piece of apparatus and is furnished with a sliding contact. It is specially designed to produce a maximum difference of potential of about 25 volt. The arrangement adopted is shown in Fig. 87. The terminals E and F of this resistance are connected to those of the meter marked R and T. The armature of the latter will rotate from right to left when the cells are being charged, and in this case the current only partially traverses the resistance, going along the path from C to B. When the cells are discharging, the direction of rotation of the meter will be reversed, and the current will flow through the whole resistance from B to A. The meter, in addition to the ordinary set of dials, has a large dial graduated in amperehours. When the cells are charging, the pointers of the small dials do not move; the pointer of the large dial, however, turns counter-clockwise and indicates in ampere-hours the quantity of electricity put into the cells, reduced in the ratio of the efficiency of the battery.

FIG. 87.

By means of the sliding contact C, the ratio of the resistances of the two branches BC and BA can be adjusted until it is equal to the ampere-hour efficiency of the battery. When this condition is fulfilled, the large pointer always indicates the quantity of electricity which can be taken out of the battery.

This system can be advantageously used with electric automobiles; and the counting train can be arranged with a contact system operating a relay circuit, which will automatically interrupt the charging current when the battery is fully charged.

These battery meters are made in three sizes for charging capacities of 100, 200, and 400 ampere-hours.

In each case the large dial is divided into 100 parts, each division of which corresponds to one, two, or four ampere-hours, according to the capacity of the meter. Similarly, each unit on the first of the small dials reads ten, twenty, or forty ampere-hours. An exactly similar arrangement is adopted by the Danubia Actiengesellschaft also with their O'K. meters.

The Siemens-Schuckert Battery Meters consist of the company's ordinary continuous current types, each of which is fitted with two separate counters and a change-over device. The one counter registers the charge and the other the discharge, and a small index on the

dial face indicates the one which is working, and whether the battery be supplying or receiving current.

The connection between either counter and the meter spindle is made by a movable lever, on the one end of which is mounted a worm and on the other a worm wheel, which gears with the worm on the meter spindle. According to the direction of rotation of the meter, in one direction on the charge and in the opposite direction on the discharge, the worm wheel causes the lever to move upwards or downwards and the worm to gear with a corresponding wheel of the respective counting train, thus bringing the latter into action. Fig. 88 is an illustration of the Peloux meter manufactured by this company, and fitted with two counters and a change-over arrangement for battery purposes. The meter is also shown with a special set of test terminals.

Section B.-Switchboard Meters.

FIG. 88.

For the measurement of the output of direct current lighting, power, and traction systems, special switchboard instruments are used. They retain the principles common to their respective types, but are variously modified to comply with the severe conditions of heavy and fluctuating loads obtaining in central stations in general. Precautions have also to be adopted to shield the meter from the influence of the magnetic fields set up by neighbouring bus-bars or heavy-current cables.

Hookham type.-Two methods are in vogue. In the one the meter is supplied with a high-capacity shunt traversed by the whole current, and potential leads connect the current circuit of the meter to the terminals of the low-resistance shunt, so that the current flowing in the meter is always a determinate fraction of the main current in the bus-bars or feeder circuit. This method is, however, mainly restricted to quantity meters. An illustration of the Hookham ampere-hour meter for 2000 amperes, with its shunt, is shown in Fig. 89, and may be taken as typical of this class of heavy-current shunted meters. The current in the meters is in this case about 50 amperes.

For switchboard work, the meter is separated from its shunt, and the latter is mounted on the back of the board, while the meter is fixed on the front.

Thomson Switchboard Meters. In the other method, a construction is adopted in which the whole current passes through the meter itself. In this manner any errors due to the use of a high-capacity shunt are eliminated. In the Thomson meters the entire current flows through the field coils, which, for heavy currents exceeding 1500 amperes, consist of a single heavy bus-bar of forged or cast copper of high conductivity. It will be remembered that in

the ordinary Thomson meter one armature connected to the pressure circuit is used. In this case, however, two armatures connected together in series are employed, and are oppositely wound. They are arranged astatically, the one above and the other below the bus-bar of the meter, so that they are cut in opposite senses by the magnetic field produced by the current in the bus-bar, and both, therefore, tend to rotate in the same direction,

The damping magnets are enclosed in a cast-iron box. This construction

ensures practical freedom from stray fields and bus-bar effects. The entire meter is supported upon the switchboard by two studs, which form at once the electrical connections and the mechanical support.

The astatic switchboard types for heavy currents of the British ThomsonHouston Company, Rugby, and the General Electric Company, U.S.A., are respectively illustrated in Fig. 90 and Fig. 91. Noteworthy differences exist between them, mainly in the construction of the armature and compounding coil, and in the number of coils used. In the meter of the British Thomson

Houston Company, not only are two armatures used, but each has its own commutator and a stationary core composed of iron washers; further, the armatures, each consisting of two coils, are in planes at right angles to one another. The compensating coil consists of a few turns of fine insulated copper wire acting on the upper armature, as shown in the illustration, Fig. 90. This type of compensating coil is, however, only used in meters intended for extra heavy currents and furnished with the square copper studs.

The meter of the General Electric Company, U.S.A., has only one commutator, and the two armatures do not materially differ from the ordinary drum-wound type; they are oppositely wound and contain no iron.

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