Electric meters are used for measuring the amount or quantity of electricity consumed in factories and houses. What electric meters actually measure is the electric energy that passes through the circuit. for direct current the energy is A = V.I.t,. In alternating current supply systems, only watt-hour meters are used.
Older meters like the electrolitic meter, used to be used for alternating current only and is now obsolete, but its mode of function is still of some interest. It consists of a cathode, an electrolyte and a scale. There is a separator at the top, and a scale is at the bottom of the meter. A closed glass vessel contains an electrolyte consisting of a solution of mercury salt. Some of the consumers current flows through this electrolyte. At the cathode a quantity of mercury is deposited. The mercury collects in the bottom part of the vessel. The scale is graduated in kWh. The mercury is then retrieved and returned to the top of the compartment.
Another older type of meter, is the motor-type meter, which is a small direct current motor. There is a disc of aluminum which is mounted within the field of a permanent magnet. The aluminum disc contains three coils, which are supplied with a current from a commutator consisting of three sectors. The current supplied to the coils is proportional to the to the total current passing through the consumer installation. Another type of electric motor meter is the induction meter and can still be seen in use today. The induction meter has no commutators and there are two electromagnets. The coil of one of these is energised by the consumer current and the other coil is connected to the consumer voltage. The interaction of the two coils produces a moving magnetic field, which induces eddy currents in the aluminum rotor disc.
These currents cause the disc to rotate in the direction of motion of the moving field, and the speed of the rotation of the disc, is proportional to the strength of the two magnetic fields. It is also dependent on the on the phase displacement of these fields and therefore upon the power factor. Nowadays most electric meters electronically controlled with the use of various circuits and small computer chips. This enables the meter to record not only electricity consumption but other information as well, like the time and date when the electricity was used, and also help to produce invoices. Electric meters are extremely common, and can be found just about anywhere electricity is consumed. Almost all residencies will have at least one electric meter installed, and this can be of several different types. For example, some meters are manufactured specifically for different residences such as an economy rate meter or a, pay as you go type meter. Pay as you go type of meters refers to electricity purchased in advance. This is achieved by the use of a payment type credit card or payment ‘key’.
The card or key is inserted into a special purpose electronic outlet, and a specified amount is paid for and the information uploaded onto the card or key. The card or ‘key’ is then inserted into the appropriate electric meter, which receives the uploaded information. The most common unit of measurement is the kilowatt hour, and the demand is measured in watts and averaged over a period of time, usually a quarter of an hour.