Fig. 1.
Fig. 2.
Charges for Electricity.
The well-known expression “per 1000 Cubic Feet” is not applicable to electric light, and, instead, the Board of Trade Unit is employed. By this term Unit is meant the quantity of energy contained in a current of 1000 Ampères flowing under an Electro-motive Force of One Volt during One Hour. In the early days of electric lighting the term Volt-ampère was used, and has for convenience sake been shortened to Watt; that is, the Volt or Unit of Electro-motive Force (or pressure) is multiplied by the Ampère or Unit of current.
The Board of Trade Unit is, therefore, a Thousand Volt-ampères or Watts per hour.
For example: 16 candle-power Swan lamps are assumed to take 60 Watts, which, if the electrical pressure is 100 Volts, would mean a consumption of 0·6 Ampère; and, as an Electrical Horse-power equals 746 Watts, 12·4 lamps should theoretically be obtained per Horse-power, which is, however, reduced in actual practice to 10 at the most, often less.
The charge per Unit supplied by meter varies in England from 1s. to 7d.