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Charges for Electricity.

Table of Contents

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.

Price for electric light.		Equivalent price for gas of equal light.
s.	d.				s.	d.
1	0		per Board		6	10	per 1000 cubic feet.
0	9	of	5	1½	””
0	7¼	Trade	4	2	””
0	6	Unit.	3	5	””

Comparison of Cost of Gas and Electricity.

These prices, of course, include the manufacturer’s profit as well as the loss in transmission through the mains and expenses of connecting up to the consumer. The actual manufacturing cost of a station maintaining 10,000 lights should not be more than 3d. per Unit, or equivalent to gas at 1s. 8½d. per 1000 cubic feet.[2]

As petroleum lamps are used for the street lighting of many foreign and colonial towns, the question arises, “Will it pay to substitute the electric light?” Comparing the light given by a kerosene or petroleum lamp with that from the incandescent electric lamp, the cost is greatly in favour of oil; and, in fact, where the price of kerosene is under 1s. per gallon, electricity cannot compete if labour is cheap. On the other hand, the trimming, lighting, and keeping in order of a number of lamps scattered over a large area greatly augments the working cost, to which must be added the breakages of chimneys, expense of wicks, also the danger of fire. It is the safety of electricity which has caused it to supplant oil both for public and private lighting in American cities; even where the price of kerosene is not more than 6d. per gallon there is a demand for the electric light, which is by far the dearer illuminant, after making a liberal allowance for labour in cleaning, filling, and lighting the oil lamp, also for depreciation of the burners.

Arc and Incandescent or Glow Lighting.

Fig. 3.

Electric lighting can be obtained by means of arc or incandescent lamps. The arc light is now well understood to be caused by the extremely high temperature of the end of one or both the carbon electrodes. The voltaic arc, Fig. 3, is formed by the minute particles of carbon in a high state of combustion which the current appears to break off and carry from one electrode to the other, the light, however, being mainly due to the incandescence of the crater shown in Fig. 3 on the upper carbon. In the incandescent or glow lamp light is produced by the passage of a current of electricity through a continuous fine thread or filament of carbon which becomes white-hot, the destruction of the filament being prevented through its enclosure in a glass bulb from which the air is exhausted. Figs. 4 and 5.

The first method is suitable for the lighting of streets where a high-class illumination is required; also will be wanted for the external lighting of shops, public-houses, and places of amusement, so that arrangements must be made for arc lighting. The usual plan is to charge at the same rate per Unit by meter as the incandescent lamps, but to make an additional charge of 5s. to 7s. 6d. per lamp per quarter for rent, and a further charge of 3s. per week for cleaning and trimming.

The principal types are the Edison and the Swan, Fig. 4 and Fig. 5.

Incandescent lamps can be obtained to order from 1½ candle-power upwards, but the 16 candle-power (nominal 20) or the 8 candle-power (nominal 10) lamps are almost invariably employed. The latter give the best effect, and can be worked to 10 candle-power without much risk, they take about 30 watts as against 60 watts for the 16 candles; and are not uneconomical, for nearly double the number can be worked with the same energy. A new type of glow lamp, called the “Sunbeam,” has been recently introduced, which contains a thick filament, and gives a light of from 200 to 1500 candle-power, and can be employed instead of an arc lamp with the same economy as the ordinary 16 candle-power type.

Fig. 4.Fig. 5.

Life of Incandescent Lamps.

In estimating the annual cost of lighting, the renewals of lamps must be taken into account; and although some lamps have worked 3000 or even 4000 hours, a life of 1000 working hours is the highest average it is safe to assume in practical work under even the best conditions, that is, using secondary batteries and never over running. The average life of 130 lamps on H.M.S. troopship Malabar was 3799 hours each, the shortest life being 638½ hours for 18 yard-arm lamps of 32 candle-power. If the current is allowed to fluctuate, the average life would be very much less; it is an unsettled question whether long-lived lamps are really economical, by reason of the blackening of the globes, which takes place after the lamp has been worked some time, and is probably due to small particles of carbon thrown off from the filament being deposited on the glass. It has been suggested that attrition of the filament is going on all the time the lamp is at work, and that the heated atoms striking against the filament may account for the blackening, in that the mean free path of the atoms would be greater in a perfect vacuum than in the air, consequently they would abrade the filament with considerable force. If lamps were sold at 1s. each instead of 3s. 6d., which is now the price for not less than a thousand, it would be more economical to change them at the first signs of blackening, even if the life did not exceed 500 hours.

The diagram, Fig. 6, has been so arranged that the amount of light required in a given district can be ascertained for any period of the day or night; it has been calculated from the observations taken daily at one of the Berlin central-stations by the engineer to the company.

Six hundred and forty watts are assumed, for the purposes of the diagram, to be the equivalent of a horse-power, instead of 736, as the German electrical horse-power is 736 watts instead of 746 watts.

Fig. 6.

The table, Fig. 6, has two vertical scales, A and B, each giving the kilowatts[3] and corresponding horse-power. A is drawn to a scale ten times greater than B, with the object of noting the smaller amount of lights required for street illumination. The horizontal line is divided into hours, and represents a day’s lighting in the middle of December and the end of July, so as to show the maximum and minimum amount of current that will be required. In the lighting of a town there are two classes of illumination, the amount taken by the public, which is uncertain, and that employed for street lighting, which is a known quantity.

The curves, II and II A, represent the private lighting of houses, hotels, theatres, and shops of different kinds in December and in July, the curve, II A, being in dotted lines clearly shows what a vast difference there is in the amount of light, and consequently the amount of energy required in the generating station, as compared with curve II, which is taken when the days are longest.

The rectangles, I and I A, show the street illumination, and are drawn to suit scale A; half an hour after sunset all the lamps are turned on, and the work reaches its maximum suddenly, and continues the same until 12 o’clock, when, according to the municipal decrees, it either falls one or two gradations until half an hour before sunrise, when all the lamps are extinguished. The calculations are based on the assumption of 640 watts to the horse-power, instead of 736, which is the theoretical efficiency of a German horse-power.

If a number of diagrams are taken on this method for different periods of the year, the constant work can be ascertained. This knowledge is most valuable when calculating the most economical area for the mains, which is then easily accomplished by means of Forbes’ tables, which are based on Sir William Thomson’s well-known rule.

The lines, 2 and 2 a show the constant work at the same two periods of the year from which the diagrams are taken. The constant work at the end of December will be found to amount to 20 per cent. of the total work, and that at the middle of June to 15 per cent. By summing up the average work for all the days in the year we obtain the cost per annum, and adding to this the expense of management, interest, &c., and knowing the local conditions, we can fix what proportion of the day’s work is admissible as loss. With the Edison system at Berlin, 5 per cent. is taken as average loss; thus, at the end of December, it amounts, with the maximum number of lights, to 18·8 per cent., and with the minimum to 1·1 per cent.; in the middle of July the maximum is 15·8 per cent., and the minimum 0·5 per cent. The dynamos must, of course, be of sufficient power to be able to overcome this loss, which only shows itself periodically; therefore the generating plant may be constructed to give, nominally, 20 to 30 per cent. less than the maximum work, and be capable of being pushed to the full amount for a short time only.