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.
The Position of Central-Station Lighting.
Uncontrolled financial speculation, aided by the stringent clauses of the Electric Lighting Act of 1882, have been a great deterrent to the extension of old or the introduction of new schemes for the supply of electricity to the public in the same manner as gas. The President of the Board of Trade, replying to a question in the House of Commons, said that, “since the passing of the Electric Lighting Act of 1882, fifty-nine provisional orders and five licences had been granted to companies, and fifteen provisional orders and two licences to local authorities. He was not aware that, in any single case where these powers had been obtained, they had been exercised.” Up to the present time no company supplying electricity has been under the necessity of applying for compulsory powers, and has either obtained permission from the local authorities to take up the streets, or has carried the electric mains over the houses, and, regardless of the question of overhead wires, has depended on wayleaves granted by the too-confiding householder, who has no idea that his roof is supporting a cable weighing 1¾ tons to the mile.
An amendment of the Act of 1882 has passed both Houses without hindrance, and has received the Royal assent. It provides that in the case of Provisional Orders the period after which the undertaking may be bought up by the Local Authority shall be extended from twenty-one to forty-two years, and that portion of the previous Act which referred to the compulsory purchase of the undertaking by a local authority at the end of the term has been altered, and more favourable terms given to the electric companies.