In Lane - Fox's quantity meter the entire current of supply is passed through the coils of a solenoid; the movable core, or plunger (the degree of insertion of which within the solenoid depends upon the intensity of the current), is made pendant from one end of a beam or balance. From the other extremity of the beam hangs a plunger, in the form of an inverted cone, which in its position of rest fills up completely, with the base of the cone, a small vertical water - pipe, the water being constantly supplied from a small cistern above. The result of the passage of an electric current through the coils of the solenoid is to suck the core or plunger deeper in, and in doing so the conical water - plug is lifted, and its diameter at the orifice of the pipe being diminished in consequence, water escapes down the pipe. The amount of this water, being directly dependent upon the intensity of the current, becomes a measure of its quantity. A cistern is provided for this overflow; and when it becomes full, it is automatically emptied. These successive operations of emptying are recorded mechanically by a train of wheels.

By this means, a record is kept of the quantity of the supply.

F. H. Varley's meter is based partly upon electric and partly upon mechanical principles, the successive increments of depositions being made visible by a pointer on a curved index; and the several reversals are understood to be alterations in the mechanical direction, and not in that of the current (as in Sprague's meters).

(6) Energy Or Work Meters

(1) Boys's

This consists of 2 parts - the indicator of energy, and the integrating apparatus. In the indicator of energy, a balanced beam has from one end suspended a counter - weight, and from the other a hollow solenoid, free to work up and down into 2 other solenoids. The movable solenoid is wound with a considerable length of fine wire, in the upper half in one direction, in the lower in the opposite (this is to render it independent of any magnet which may be placed near it). This solenoid constitutes the high resistance shunt which measures the E M.F. The 2 fixed solenoids are wound with thick wire, and convey the main current. The result of the action of the fixed and movable solenoids on each other is a force proportional to the product of the 2 currents, that is, to the energy expended; but the external evidence of this is the inclination of the beam, and this inclination, or rather the tangent of the inclination, is proportional to the energy being expended.

The recording apparatus consists of a cylinder, which, by . means of a mangle - motion, is made to reciprocate backwards and forwards by clockwork, and during its passage in each direction the cylinder is made to bear alternately against one of 2 tangent wheels, each free to be inclined in its direction of travel: both are fixed on the same swivelling frame, but only one of them bears at the same time against the cylinder. This frame is free to be inclined from the vertical in correspondence with the inclinations of the beam. The effect of this inclination of the tangent wheel is to cause the reciprocating cylinder to rotate, the speed of such rotation being proportional to the tangent of the inclination of the wheel, which is likewise proportional to the tangent of the inclination of the beam, i.e. to the amount of energy expended. The path of the tangent wheel on the reciprocating cylinder, when not inclined, is simply a straight line lengthways along the cylinder, and no rotation is caused; but when, owing to the inclination of the wheel, the cylinder rotates, the wheel - path becomes a spiral.

The rotations of the cylinder are transmitted to a train of wheels, and registered, thus giving a record of the amount of energy expended during a given time.

(2) Deprez's instrument consists of a thick wire coil, movable upon its axis, which passes through its centre of gravity. This axis - shaft is set upon 2 knife - edges, insulated from each other, and communicating respectively with the coil; and with a metallic bar having a curved, quadrant - shaped head, which dips into a mercury cup, and thus forms connection with the main current. The shaft also carries, projecting downwards, a pendulum rod, the bob of which is intended to cause an antagonistic force; while the extension upwards of the rod forms a pointer, with a scale fixed on the frame of the apparatus. On the frame there is also a fine wire coil, on a shunt, fixed so as to surround the movable coil. The deflections on the pointer are therefore due to the product of the intensity of the current and its electro - motive force; that is, to the power developed. The readings of this instrument may be integrated either by a Deprez rotating disc and ball integrator, or by one designed by Abdank (of Cracow), having a travelling cylinder and tangent - wheel arrangement, somewhat similar to that of Boys.

(3) Ayrton and Perry's erg - meter is but a further development or sequel to their power - meter, by the addition of apparatus which integrate and record continuously the time during which the electrical energy has been imparted, as well as the variations in its amount. By this means is preserved a record of the entire work done, or of the total electrical energy supplied. As in the power - meter, 2 coils are here made use of. There is a thick wire one on the main circuit, to measure the amount of current; and a thin wire one on a shunt, joining the ends of the main circuit, to measure the difference of potential, or electromotive force, of the main circuit. The thin wire coil, of say 1000 ohms resistance, simply replaces the pendulum bob of a clock. The wires from each end of the coils pass up the sides of the pendulum rod and on to binding - screws, which can be joined to the supply and return cables of a house, or machine, or a system receiving electrical energy. In the immediate vicinity of the fine wire coil, fastened to the clock - case and parallel with the plane of the pendulum path, is fixed the thick wire coil, which forms part of the main circuit and has a very small resistance.

The effect upon the thin wire coil of its repeated passages in front of the thick wire coil is to cause a certain pull or attraction upon its motion - either of acceleration or of .retardation, according to the direction of the coiling. This action, in addition to the ordinary action of gravity upon the pendulum, will keep constantly adding to or taking from its rate of motion, in proportion to the electrical power of the circuit. This pull is the product of the magnetic moments of the 2 coils, and therefore is proportional to the product of the current and the electromotive force. The effects of these repeated accelerations or retardations upon the progress of the clock keep constantly accumulating, and their total amount can at any time be detected and ascertained by observing the amount of loss or gain which the clock has experienced. As the rate of loss or gain in the clock due to different amounts of electrical power has been previously ascertained, this knowledge of the total retardation or acceleration upon the clock is, in fact, a record of the total amount of electrical energy which has been expended, or of the work done, since the last observation of the clock.

Prof. W. Grylls Adams observes that before much can be said about these various instruments, they require further trial and comparison with one another. One important point is economy; a simple instrument will have the preference over another which may be equally effective but more complicated. If these instruments are to be used by every consumer, it is a matter of great importance that they shall not be unnecessarily expensive.

Preece advocates the adoption of a mode by which lighting may be paid for, not by measuring the quantity of electricity consumed, but by the measurement of the amount of light given as compared with that given by 1000 cub. ft. of gas. What is supplied and used is light, and what people want to pay for is light; 1000 cub. ft. of gas give a very convenient standard, and if the electric light companies could only formulate some plan by which they would charge accounts for the light supplied, he believes the public would be more ready to use it than if they were charged per volt - ampere, or any other measurement which a few understood but the majority did not. The most' interesting subject is, not how electricity is to be measured, but how it is to be paid for. Boys's instruments are perfect of their kind, and Dr. Hop - kinson's instrument for measuring the quantity of electricity passed into a house, though very little known, is one of the most perfect things he has ever seen. Why he objects to pay for electricity is because he believes we do not know how much light it will produce.

If we agreed to pay 7d. per 1000 volt - ampere hours this year, the probability is that next year such improvements would be made, that the same energy which now produced 200 - candle power, would then produce 2000.