These may be divided into 2 classes - (a) Quantity or coulomb meters; (6) energy or work meters.

(A) Quantity Or Coulomb Meters

These are separated into those based upon electrolytic action, and those which are mechanical in principle.

(1) Electrolytic

Edison's current - meters are based upon electro - deposition of metal, due to the action of a known fractional part of the total current. The weight of the increments is ascertained periodically, and from it the total quantity of the current which has passed through the interval is deduced. The metal used consists of plates of amalgamated zinc, immersed in a solution of 90 parts zinc sulphate and 100 of pure water. In the form of meter for commercial use, 2 cells are placed as a check against each other; one, termed the " monthly cell," receiving 4 times the current of the other, which is known as the "quarterly cell." To prevent the temperature of the liquid in the cells falling so low as to freeze, connection is made, by means of a long thin strip of brass and steel riveted together, to an incandescent lamp, which is thereby lighted, and raises the temperature as required. It is only when the temperature falls to 42° F. (5 1/2° C.) that this tongue is sufficiently depressed to form contact, and so to light the lamp. On the temperature rising, the tongue rises, and the lamp is extinguished.

Experience shows that electro - deposition, to give a true and reliable record, should not be forced or overworked in its action; and that the plates should not in their daily duty be required to do more work, or be longer in action, than they are intended for by their superficial area. In practice, about 75 Per cent. only of the nominal work should be required of them. It appears to be the custom to design the duty of a meter for 3 hours. burning of each lamp per night. Thus, a 25 - light meter would have its plates designed for 75 lamp - hours per night. As just stated, it is not advisable in practice to exceed 75 Per cent. of this amount. However, it is seldom found that the whole of the lights are used for the entirety of the time allotted to each; and if this were so, it would merely require a larger meter to meet the case. It is said that over 300 meters on this principle are in use in New York.

Sprague's meters are based upon electro - deposit up to a certain point; ie. when the intended quantity of metal, whether copper or zinc, has been deposited on the plate. The current is then reversed, and the metal is gradually dissolved again until the primary condition of the plate is reached; when, by another reversal of the current, deposition again commences. Each reversal of the current is recorded by a mechanical counter and a train and wheels. Not much practical experience has been obtained with these meters; but what has been done tends to point out that the mechanical operations involved in the reversals of the current, and in their registration, absorb a large amount of power.

(2) Mechanical

Hopkinson's current - meter consists of a thick wire coil, in the form of a solenoid, through which the current passes to be measured. The iron core of this solenoid revolves with its central shaft by the action of the armature of a small dynamo machine placed at one end of the shaft. The core of the solenoid is in 2 parts; the lower is fixed to the shaft, while the upper is movable, being attached to a governor - ball arrangement, and sliding up and down the shaft in accordance with the variations in the rotation speed of the shaft. A shunt current passes through the dynamo and its armature, up through the lower or fixed portion of the core, (by contact only) to the sliding part, and thence to the framework of the apparatus. If the movable' core be lifted, owing to the speed of rotation, by the action of the governor - balls, this circuit is broken, and the shunt current through the dynamo is interrupted. Whenever a current to be measured passes through the coil, attraction, by means of its casing, takes place between the fixed and the movable parts of the iron core.

This magnetic action, which is proportional to the square of the current, tends to keep the 2 parts of the core together and in contact; while the centrifugal force of the governor - balls, which is proportional to the square of the speed of revolution, tends to break the contact by lifting the movable part. These opposite forces will, in working, balance one another, and the result is that the system revolves with a velocity proportional to the current through the coil. As the revolutions of the shaft are transmitted continuously by a train of wheels to a set of index dials, a record is kept of the quantity of the current that has passed.

Boys's Quantity or Vibrating Meter is based upon 2 well - known principles : (1) The force acting on the armature of an electro - magnet, in any position, is proportional to the square of the current; (2) the square of the number of vibrations, say, of a pendulum, is a measure of the controlling force. Therefore, if the controlling force under which a body vibrates is due to the action of an electro - magnet on its armature, the square of the number of vibrations in a given time is a measure of the square of the electric current. In other words, the rate of vibrating is a measure of the strength of the current; and the number of vibrations is a measure of its quantity. The exact form and nature of the meter may vary in many details. One form consists, primarily, of an electro - magnet (the upper one), through the coils of which passes a portion of the main current to be measured. This magnet is placed horizontally, and a vertical rocking shaft stands between its poles. This shaft has fixed on it a soft iron armature, rounded at the ends, and free to move in the horizontal plane between the poles of the electro - magnet.

The intensity of the attraction between the poles and this armature determines the rate of vibration, which, as above stated, is a measure of the strength of the current. Each vibration is itself recorded by means of an escapement, a train of wheels, and a set of index dials; and the number of vibrations thus registered becomes a measure of the quantity of the current. To add to the momentum of the vibrating body, 2 long arms, weighted at the end, are attached to the lower part of the vertical shaft. To prevent the vibrating armature from gradually coming to rest, it is arranged that, when the vibrations fall below a certain limit, by making contact, a portion of the current is sent round the coils of a second or "impulse " electromagnet (placed underneath the "controlling" magnet), and which has an armature of a suitable form fixed on to the same shaft that carries the armature of the upper magnet. The extra motion thus given to the shaft by the attraction of the lower armature, affords the necessary impulse to the vibrating armature when required.