the speed be too high, the screws must be screwed out, further away from the poles. This affords a very reliable and easy method of effecting the high load adjustment. The screws are fixed by means of lock nuts when the adjustment has been completed, and the magnet itself is not moved. The friction compensation device, by which the speed of the meter is regulated on low loads, consists of an iron strip and screw shown at F. This strip shades the left-hand pole and produces a shifting field which tends to drive the disc. The intensity of the field is controlled by the iron screw, which, on being screwed in, accelerates the disc.

The phase adjustment for inductive loads has already been explained, and is performed by altering the position of the sliding contact X, which is soldered when the adjustment is made. By altering the position of X the resistance in the circuit H is changed, while that of the circuit H' remains unaltered. This varies the relative reactance of the two branches. If the resistance in H be increased, 2 will increase and 4, will diminish (Fig. 170). X is moved up or down until the resultant series flux is at right angles to the shunt flux, when the meter is correct. This adjustment is usually carried out with a power factor of about 5 or 3. The shunt loss is very low, generally less than 0.5 watt, from 100 to 230 volts, with a periodicity of 50 cycles per second. The torque at full load is 3 centimetre-grammes, and the weight of the revolving element 60 grammes.

Single-phase Induction Meters, Types B.L. and I.R.-The alternating current meter for inductive loads, type B.L., of the Compagnie Anonyme Continentale pour la Fabrication des Compteurs, Paris, is illustrated in Figs. 173 and 174. The requisite quarter-phase displacement between the main and the pressure current fluxes operating the meter, when the main current is in phase with the pressure, is obtained by the employment of a suitable reactance in series with the pressure coils and an inductive shunt to the main windings of the meter, which latter are, further, connected together through an adjustable resistance. The main supply current, on entering the meter, splits up between these two branches. The inductive shunt will only carry a small fraction of the whole current in the circuit, owing to its self-induction, and this current will lag behind the total current, whereas that flowing in the main windings proper of the meter will form the larger part of the total current, and will lead in advance of it.

By suitably adjusting the resistance, which alters the relative reactance of the two main current branches, and also the choking coil in the pressure circuit, the meter should function correctly on inductive loads.

It can be easily shown that the current in the main windings will always be proportional to the total main current flowing in the circuit, whatever the value may be. The object of the choking coil in shunt with the main current coils is to reduce these windings, and to cause the current in them to lead and to produce a leading magnetic field. It will be further noticed that the flux due to the lagging part of the current in the inductive shunt is not used to influence directly the revolving part of the meter.

Referring to the illustrations, an aluminium bell C constitutes the armature of the meter, and is mounted on a steel spindle A, resting on a flexibly carried jewelled bearing. The upper part of the spindle carries both the driving worm actuating the meter dials and the aluminium brake disc P, rotating between the poles of a permanent magnet E. The driving element is composed of a laminated iron ring having three inwardly projecting poles N, N', N". The bell C rotates in the air-gap between these poles and the

eccentrically-placed laminated iron core M. The core M serves to concentrate the lines of force and to augment the driving torque. By means of the adjusting arm m attached to it, the core can be displaced relatively to the poles and the sensitiveness of the meter adjusted on low loads. The pressure winding consists of three coils mounted on the three poles, as shown in the illustrations. In series with these coils is the adjustable choking coil K, and the pressure circuit is connected direct across the supply mains.

The two main current coils are wound on the poles N and N', and are connected together by the resistance R, which can be partially short-circuited by the adjustable sliding contact r. These coils are connected at their other

ends to the main current terminals bb', which are shunted by the inductive resistance SS.

The speed of the meter at top loads is increased or decreased by altering the position of the brake magnet E, adjustably mounted for this purpose.

A meter suitable for small consumers is also manufactured by this company. A sectional drawing of the instrument, type I.R., is given in Fig. 175, and its action will be understood by reference to Fig. 176. The moving element is a copper bell C mounted in the usual way; it forms at the same time the brake in conjunction with the permanent magnet E. The shunt coil of the meter is wound on the yoke of a laminated iron magnet, the two pole-pieces of which are each provided with three teeth. The copper bell rotates between these pole-pieces and encloses a stationary soft iron core M. The core concentrates the fields produced and forms the light load adjustment

device. Its position relatively to the teeth of the poles is altered by rocking the adjusting arm D, which is attached to it and is controlled by means of the two screws V and V'. A very sensitive adjustment can be obtained by this method. The main current coils are distributed in the notches on the polepieces, as shown in the diagram, Fig. 176, and are wound in the same sense on the two branches of the magnet. The result is that the fluxes produced by these main current coils oppose each other in the iron core; they, therefore, form local circuits round the windings. A laminated iron bridge-piece P

N

donation

H

FIG. 176.

short-circuits the two limbs of the magnet and considerably increases the selfinduction of the pressure coil connected in series with the choking coil S.

The greater part of the shunt flux h takes the path through the bridge P, and only a small part traverses the core M. This part, together with the magnetic flux H of the main current coils, produces the driving torque.

The shunt flux h and the main current flux H are indicated respectively by the dotted lines and those shown in full in Fig. 176.

Eclipse Meter. Fig. 177 is a diagram showing the motor system used by the Luxsche Industrie werke, Munich, Germany, in their 'Eclipse' meter, type F.E.G., for non-inductive loads only, such as incandescent lamps. The shunt flux is produced by means of a single pressure coil S wound on the central limb of a three-pole laminated shunt magnet N, constructed as a

choking coil, with an almost closed magnetic circuit. About 90 per cent. of the lines of force form closed loops through the vertical air-gaps V, and V2, and the remaining 10 per cent. cut the revolving disc A and induce the shunt eddies in it.

The lag of the shunt current behind the pressure of the circuit is about 75 degrees, and an auxiliary choking coil is not used for circuits up to 250 volts.

Below the armature disc A and opposite the shunt magnet N is the four-pole series stator, which is energised by two main current coils wound on the two inner poles P, and P, and placed in series in one of the supply mains. With the polar gaps of the shunt and series magnets displaced relatively to one another, as in the arrangement adopted, the shunt eddies are dissipated in the

2

3

main current field and the series eddies in the shunt field, and, in consequence, the torque obtained is fairly high compared with the relatively weak fields used.

For circuits in which the current is out of phase with the voltage, this company use the 'Eclipse' meter, type F.E.M., in which an exact quarterphase difference is obtained between the shunt and main current fluxes when the power factor is unity.

The method employed is similar to that adopted in the A.C.T. meter, and consists in making the main current flux lead in advance of the main current. The main and series system of the type for inductive loads is illustrated in the diagram in Fig. 178. It differs from Fig. 177 mainly in the series winding, which is composed of four coils wound on the four poles of the series magnet. The main current, supposed in phase with the P.D., divides into two components C, and C, of which the one, C,, flows round the two inner poles P2 and P3, giving rise to only a few lines of force, which nearly all cut