The diagram, Fig. 1, taken from a London residential district, shows how the number of lamps on at one time vary; the district is supposed to be wired for 10,000 lamps, and the plant as equal to the supply of 600 kilowatts, or 600,000 watts; the number of lamps is small until about 3 o’clock, when it gets dusk on a winter afternoon; it then increases steadily until about 6.30 o’clock, when the curve goes up with a rush; about this time a great number of people are preparing for dinner, and probably the lights are on both in the dining-room and bedrooms. The curve falls, and at about 8 it begins to sink gradually until 10 o’clock; a great many people appear to go to bed about this time, but a few sit up to 1 o’clock; until 6 the next morning hardly any supply is taken, when the servants get up and prepare the rooms for the day. The diagram, Fig. 2, is taken from the Edison Company’s central-station at Cincinnati, and agrees fairly with the London demand for light. Another interesting fact has been ascertained from the observations taken at the Mauer Strasse station in Berlin, namely, that the output varies with the temperature, it goes up or down with the thermometer. The reason is easily explained; gas is laid on side by side with the incandescent lamps, and the burners are first lighted when it is cold to warm the apartments; in warm weather the electric light alone is used. From these and other diagrams the very important fact has been obtained, that the average daily output of a station throughout the year is less than one-third of the total capacity of the generating machinery, so that, although the station from which the diagram in Fig. 1 was taken could maintain 10,000 16 candle-power lamps simultaneously alight, the average daily output of electricity would only equal 3500 constantly lighted; and, as the first cost of the station is dependent on the size of the plant, the saleable output is the important factor which governs the profits. [Illustration: FIG. 1.] [Illustration: 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.