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nary rates. It is necessary, however, to install it in order that the business may be held for the rest of the year. Various attempts have been made to take account of these points in charging for energy, and also to encourage its use at those hours when it can be delivered more economically. The several plans for selling electrical energy, some of which take account of these conditions, are as follows : –


1st, Contract to supply a certain number of lamps at a fixed price per month,

whether they are used or not. 2d, Meter with “flat” (i.e., uniform) rate. 3d, Meter taking account of maximum demand. 4th, Meter, with two or more rates of charge for different periods of the day. 5th, Fixed charge for energy plus a graded charge for the maximum capacity. 6th, Prepayment meter, which only allows energy to be delivered for a certain

coin deposited.

The contract system of charging a fixed amount for a certain number of lamps was commonly adopted in the early days of electric lighting, except in Edison systems using the chemical meter. For street lighting, and other service requiring lights for a definite time, this system is satisfactory, but for residence lighting it is quite unsatisfactory, because the consumer is likely to burn the lamps for the full time when they are not needed. This wastes a large amount of energy, which must ultimately be paid for by the users. In such cases, and in fact for general use, some form of meter should be adopted. The common plan is to charge a certain price per lamp-hour or k. w. hour, which is graded according to the amount used. For example, a common practice is to charge one cent per lamp-hour, if the consumption is equivalent to the full number of lamps burning for one hour per day; i cent if equivalent to two hours, and so on. This accomplishes its purpose fairly well, but fails, however, to take into account the particular time at which the lamps are burned. The latter point may be covered by using a two-rate meter, which separates the energy consumed during certain hours from that used during the rest of the time, a higher rate being charged for the former. One way of accomplishing this is to use two separate meters which are switched in or out of the circuit by clock-work. Another plan is to cause the meter to run faster during the time that a higher charge is to be made, thus arriving at the same result. The prepayment meter is not intended to accomplish any of these ends, but is simply to avoid the necessity for giving credit.

The arbitrary method of charging a certain price per unit for one quantity, and t as much for a greater quantity, is objectionable, because it leads to the absurd result that one may reduce the amount of his bill by using a little more current. The slidingscale, in which the reduction is a certain percentage of the amount used, would avoid this difficulty. It might not be quite so easily understood as a certain price per lamp-hour, but customers would soon understand this plan. It would seem to be practically impossible to make a perfectly fair arrangement between producers and consumers; but a reasonable approximation can be reached, which is close enough for ordinary business purposes.

The Edison Chemical Meter was the first successful type, and many thousands of them were in regular and satisfactory service for a number of years. For reasons, given later, they have been replaced by other forms operating mechanically. In principle, this meter is based upon Faraday's law, according to which the amount of electrochemical action, for example, the weight of metal deposited or dissolved in an electrolytic cell, is directly proportional to the current, the chemical equivalent, and ihe time. The ampere, as legalized in all important countries, being defined in terms of the weight of silver deposited, this principle is fundamentally correct. For reasons of cheapness and as a result of numerous experiments, Edison adopted zinc as the best metal for the purpose.

The meter consists of a cell C, containing a solution of zinc sulphate having a densiiy of 1.11, in which two zinc electrodes, A and B, are immersed. These are kept parallel and at a fixed distance apart by hard-rubber bolts. Connection is made to them by copper rods inserted in their upper ends, as indicated. In one of the main conductors, + or – which supply the lamps L, whose current is to be measured, a german-silver shunt S is introduced, having a certain resistance that is practically constant for ordinary temperature changes.

The electrolyte in the cell has a certain resistance, which decreases with rise of temperature; and in series with it is a coil R of copper wire whose resistance increases with temperature, the two being proportioned so that they compensate each other and keep the resistance of the cell circuit almost perfectly constant for ordinary changes in temperature. The resistance of the coil R is about 4 times that of the bottle, and the two together have many times the resistance of the shunt S, so that a certain small fraction of the total current passes through the cell, dissolving zinc from the anode A and depositing it upon the cathode C. Once each month the electrodes are removed from the cell, being replaced by others, and the loss in weight of the anode is carefully weighed by a chemical balance. This loss in grams multiplied by the ratio of resistances of the two branches of the circuit and divided by .000337, the electrochemical equivalent of zinc, gives the total number of ampere-seconds during the month.

The possible sources of error are due to temperature changes, which are almost perfectly compensated in the cell circuit as already explained, and only vary the shunt S about } per cent for 15°C above or below the normal. The cell thas a certain counter E.M.F. of .001 I poove to.003 volt, which introduces an error at loads less than 3 per cent of the maximum. Oxidation of the plates also occurs and is allowed for, otherwise the loss in weight would appear

Fig. 379. Edison Chemical Meter. too low. The drop produced in the main circuit is small even at full load, being only about %.

With reasonable care this meter is fairly accurate; but the trouble of collecting and weighing the plates, and the fact that the consumer cannot read the record himself, has lead to the substitution of more convenient forms.

Thomson Recording Wattmeter. — This type, developed by Professor Elihu Thomson, and used in very large numbers in this country and abroad, is essentially an electric motor. The general appearance of the standard two-wire form for direct or alternating currents is shown in Fig. 380, and the connections in Fig. 381. The field magnet of the motor consists of two stationary coils of heavy wire directly in series with one of the main supply conductors, so that the entire current to be measured passes through them. An armature provided with a winding of many turns of


fine wire and having a resistance in series with it is connected across the circuit between the main conductors, and is mounted to rotate on a vertical axis between the two field coils. The armature is equipped with a miniature silver commutator and with brushes similar to those of a direct current motor, but, like the field coils, does not contain any iron core. Since the magnetic circuit passes through air and other non-magnetic materials, the flux through the armature, though small, is directly proportional to the main current. The armature circuit having a constant high

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resistance connected across the two supply conductors, takes a current exactly proportional to the voltage between them. Hence the torque of the motor is in proportion to the product of these two currents or to the number of watts supplied at any instant. A copper disc mounted upon the shaft of the motor revolves between the poles of permanent magnets, as shown in Fig. 380, and acts as a brake, owing to the Foucault currents generated in it. These currents being directly proportional to the speed, the armature will rotate twice as fast with twice the

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torque; consequently the revolutions per minute are directly proportional to the power supplied in watts. The total number of revolutions in any given time represents the energy in watt-hours. A train of wheels operates a series of five dials representing 1,111,100 units, usually watt-hours, and readings taken periodically show the energy consumed during the intervals.

It is evident that a motor meter requires a certain current to overcome friction, and would fail to record any current below this limit. In order to overcome this difficulty, an auxiliary field coil of fine wire marked “shunt” in Fig. 381 is put in series with the armature. Since the latter is connected across the main conductors, the current through it depends solely upon the voltage whether any lamps are burning or not. The resistance of the armature circuit is so adjusted that this current passing also through the shunt coil develops a torque almost sufficient to overcome friction; hence any current flowing in the main circuit and field coils will produce its full effect in rotating the armature. If the armature current is too strong it will cause slow rotation, even when no lamps are burning, and this “creeping” should be

Ć Fig. 381. Connections of Two-wire Meter. stopped by reducing the number of turns in the shunt field coil until the armature is not quite able to turn when no current for lighting or other purposes is being used.

The accuracy of the meter also requires that a certain number of watts supplied shall produce the proper number of revolutions per minute. Ordinarily 60 watts should cause the armature to rotate once per minute, and so on for other loads. Measuring the power with a wattmeter, or putting on a known load of lamps, enables the accuracy of the meter to be tested by counting the revolutions per minute. If found incorrect, the speed may be lowered by setting the permanent magnets farther out, or vice versa, an adjustment of about 16% being possible. Since a reversal of current in both field and armature of a motor does not change the direction of rotation, an alternating current may be measured by this same instrument. The absence of iron cores




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