free to move 1, 2, 3, up to 6 teeth, according to the position of the governor tip. This position depends upon the extent of the attraction of the armature Rinto the coil S, and, therefore, on the number of lamps in circuit.

When no current is flowing, the lever H is held out of reach of the cam N by the governing lever, and consequently does not move. Although the pendulum bob T continues to swing, no movement of the pointer M on the face dial W can take place.

During the operation of the meter the indicator is gradually being brought back to the zero position; the moment it returns to zero, the circuit switch D is opened by the pin Z on the dial hand shaft, coming into contact with and forcing away the lever with the catch Y. No current can then be taken until further prepayment is made.

The face dial W, in the front of the meter, reads in lamp-hours, and indicates at every movement the number of lamp-hours which stand to the credit of the consumer. A check dial X is fixed on the main wheel of the recording train, and can be read through a hole in the front dial. It shows the number of lamp-hours consumed, and forms a check on the collector.

The coins found in the till multiplied by the lamp-hours per coin should equal the sum of the check and face dials, less the previous readings. In the event of the rate of charge per unit being altered, the number of lamp-hours per coin can be increased or decreased without displacing the meter from its position by increasing or decreasing the arc of contact of the adjustable sector E with the disc F.

The meter is calibrated by cutting the notches in the oscillating lever to correspond with the movement of the armature, which is centred in jewels.

At V will be seen the coin trap used, which effectually prevents the insertion of a suspended coin, wire, or a coin of abnormal proportions.

CHAPTER XII

TARIFF AND HOUR METERS.

Definition of Maximum Demand Indicator-Aron Maximum Demand Indicator-AtkinsonSchattner Maximum Demand Indicator Fricker Maximum Demand Indicator— Maximum Demand Indicators of the Reason Mfg. Co., Brighton--General Description of Two-rate Meters-Deutsch-Russische Tariff Meters-Electrical Company's Two-rate Meters-Hookham Two-rate Meter-Siemens-Schuckert Double-tariff Meters-Aron Two-rate Meter-Two-rate Meters of the Compagnie pour la Fabrication des Compteurs, Paris-Double-tariff Instrument of the Luxsche Industriewerke-Hour Meters.

Definition of Maximum Demand Indicator. In the present chapter, descriptions are included of some representative types of maximum demand indicators, double-tariff or two-rate meters and hour meters. The principles involved have already been discussed in Chapter X.

A maximum demand indicator is an instrument which registers the maximum current taken in an installation during a given period, and its scale shows, at the particular supply pressure, the number of units, corresponding to each maximum current so taken, that must be consumed during stated periods before a reduction is made in the price per unit. Maximum demand indicators are based on either the thermal or electro-magnetic effect of a current, and on the principle of intermittent integration. A demand instrument should only register the excess current when that current has been flowing for an interval exceeding the time lag of the instrument, which should be large and depend on the nature of the installation, that is, whether the indicator is connected to a purely lighting circuit or is operating on a load comprised of motors. Increments of current of short duration, as, for instance, switching on a few extra lamps for a few minutes, or temporary short-circuits, should not cause an increase in its registration.

If a steady current of A amperes be flowing in a circuit, the indicator will, of course, show A amperes, but if the current be increased to A + B amperes, and the increment, B amperes, only lasts a few minutes, then the indicator should not record more than the original current.

The Aron Maximum Demand Indicator. By means of an ingenious and slight addition to the ordinary Aron meter, it is made to function both as a meter proper and as a maximum demand indicator. The same instrument will, therefore, not only register the units consumed in the usual manner, but will also give the maximum energy taken during a given time interval, and, consequently, the units that must be consumed during that interval before a rebate is made.

It will be remembered that the differential clock of the Aron meter is driven by means of a small motor, and that the oscillation differences of the two pendulums, produced when current is taken by the installation, are

integrated by the clock and given on the dial, the indications of which represent the energy consumed over any desired period. The power of the driving motor thus available is utilised to drive an additional train of wheels, by means of which a pointer is intermittently brought into gear with, and is, in turn, actuated by, the integrating train of the meter. The position of the maximum demand pointer on its dial is determined by the angle through which the first wheel of the counter has turned in a given time. Fig. 242 is a front view of the instrument, showing

very clearly the maximum demand dial above the springing integrating counter. The maximum demand pointer is loosely mounted on its spindle, which, at the back of the dial, carries a toothed wheel. This wheel engages with the pointer by means of a pin attached to the latter, and is driven from the integrating train.

The commutator axle of the meter drives through a toothed wheel an intermediate set of wheels, which actuate a pivoted lever, on which is mounted another small wheel, which forms the connecting link between the integrating train of the meter and the toothed wheel on the pointer spindle. This small connecting wheel is thrown into gear once every twenty minutes, but only remains in gear for half this interval. This period of twenty minutes can, however, be increased or decreased to any extent. The integrating train and the demand indicator are thus in gear during a definite time, and when the former is actuated, on the passage of a current in the main coils of the meter, the pointer on the maximum demand dial will be moved during this time at a rate dependent on the energy consumed. The position at which it ultimately arrives at the end of the period will be a measure of the maximum demand in units.

FIG. 242.

At the termination of this time the small connecting wheel above referred to is thrown out of gear. The pointer will now remain stationary, and a light hairspring, which is wound up on the driven wheel of the pointer axle as this wheel rotates, unwinds and quickly returns the wheel to its zero position, leaving the pointer behind. This wheel will be again driven as already explained. Before, however, it can move the demand pointer, it must travel round until it again engages with the pin of the same. This will only happen provided that the demand during the new period is in excess of the former one, when the pointer will move further round and indicate the new maximum. To re-set the demand dial, it is only necessary to turn the pointer

back to the zero position. In addition to the advantage of one instrument giving both the actual units consumed and the maximum demand, however small, the law of the meter being a straight line law, the demand mechanism, as it is driven by the meter, is strictly proportional throughout the range of the latter.

The Atkinson-Schattner Maximum Demand Indicator, the latest type of which is illustrated in Fig. 243, works on the electro-magnetic principle. It is simply a gravity control ammeter of the solenoid type with a soft iron core, and is provided with a special time-lag registering device. The solenoid is bent into an arc of a circle and is fixed in the case on the left-hand side. The circular core is composed of thin iron sheets, increasing in number by gradual steps within the solenoid.

These iron plates are insulated from one another with a coating of varnish to prevent eddy current loss when used on an alternating current circuit, and

the section of the armature core is graduated to obtain as uniform a pull as possible. The core is carried on the lower arm of a light aluminium frame, pivoted at the centre, and shaped in the form of a sector above the horizontal axis, round which it can swing. The top portion of the frame supports the registering device. This consists of an hermetically sealed glass tube affixed to an aluminium scale. The form of the tube is readily seen in the illustration; the tube itself is filled with a viscous fluid, and contains a number of steel balls in the circular portion. The curved part of the scale is graduated to read in amperes, and the current passing is indicated at any moment by the division of the scale below the pointer fixed in the top of the case. The scale has marked on it three vertical columns of figures, in the second of which the figures denote the amperes, corresponding to the number of balls given in the first column, which have passed round the bend of the tube into the radial limb. and the third column gives the units which have to be consumed during the quarter, at the supply voltage and maximum current, before any rebate is made.

The action of the instrument is readily followed. On the passage of a current through the solenoid the armature is attracted into it, and the whole frame is tilted round the horizontal pivot. The highest part of the circular portion of the tube is moved downwards to the right, and any ball beyond this point slowly slides down the tube until it passes the bend, and collects in the radial leg. The speed of travel of the ball is limited by the viscosity of the liquid (oil or glycerine) used. The number of balls in the lower limb depends upon the angular deflection of the whole frame, and, consequently, upon the current in the main circuit.

The duration of the current must, of course, be sufficiently long to enable the ball or balls to slide past the bend. Temporary short-circuits and vibration will not affect the instrument, and, due to its sluggishness of action, it will not register increases of current lasting only for a short interval of time.

To prevent loss of time in re-setting the tube, a duplicate scale, with a second tube fitted to it, is provided. When the reading has been taken and the indicator is to be re-set, the tube is detached from the frame by means of its scale, and is replaced by the second set. The one previously in use is then hung up on the pivots in the meter cover in the manner shown (Fig. 243),

A

FIG. 244.

B

so that the balls return to the starting position ready for use. The scale is fixed to the aluminium frame by a hook at the top and a slotted pin at the bottom, to which it is locked by a small lever.

The Fricker Maximum Demand Indicator depends on the thermal effect of a current. The registration of the maximum demand is effected by means of a simple differential air thermometer, illustrated separately in Fig. 244. It consists of a tube of uniform bore, terminating in two hermetically sealed bulbs, of which the horizontal one A is cylindrical, and is the bulb to which heat is applied, and the other B is vertical and spherical in shape. At the end B the bore of the tube is closed by means of a globule of mercury, which acts as a valve; and to keep the surface of the mercury clean, hydrogen is used as the thermometric substance.

When a difference of temperature is established between the two bulbs, produced by a current flowing in the heating coil with which the bulb A is surrounded in the instrument, the enclosed hydrogen gas expands and escapes past the mercury valve into the vertical bulb B. After cooling has taken place, the gas contracts and draws the mercury thread along the tube from B towards A by an amount corresponding to the exact transference of gas from the heated bulb during the passage of the current. When the instrument is cold, i.e. no difference in temperature exists between the two bulbs, either on the discontinuance of the current, or when the tube has been removed from the instrument and cooled, the ultimate position to which the mercury