THE ABSOLUTE ECONOMY OF ELECTRIC LIGHTING.

By ROBERT BRIGGS, C.E.

The following article is extracted from the pages of Engineering, Oct. 18, 1878, where it appeared in consequence of its being a part of a discussion before the British Association where the experiments on the electric light made by the Franklin Institute in 1878 were referred It has been republished at this late day in our pages so that the JOURNAL shall include in its records the fullest statement of the condition of the art of electric lighting at the time when the experiments were instituted.

to.

It is probable, at this time, the best adjustment of quantities of light to dimensions of coils and of wire and to the power expended, will give maximums of light produced per horse-power much in excess of those established by the committee. But the direction of inquiry and the course of argument in considering the results can never differ from the investigations and tenor of the report which the committee and the accompanying paper have established.

"The remarks made by me in the discussion of Mr. Shoolbred's paper on electric lighting, which he presented at the recent meeting of the British Association for the Advancement of Science, can be properly supplemented or extended by an inquiry into the relation of the expenditure of heat in the production of light by means of the burning of coal gas on the one hand or by electrical force on the other. These relations can be looked upon from several points of view. The first of these would be the purely theoretical one, and is based on the absolute heat which would be evolved under the condition of perfect combustion and absolute absorption of heat of coal gas in producing a given light, as compared with the heat which represents the same light as derived from the power expended in producing it.

"There is produced the light effect of one standard candle by the combustion of one-third of a cubic foot of coal gas each hour. That is, this result is that of the tests for quality of coal gas as established by law, but in practice of gaslighting with the usual imperfect burners not much above (if so much as) one-half the quantity of light is obtained. Accepting this quantity it can be asserted that the heat

effect of a cubic foot of coal gas is 690 heat units (pounds of water heated one degree Fahrenheit), and that a candle power of light is represented by 230 units.

"The Franklin Institute experiments did not give the light power claimed by most makers of dynamo-electric machines; they gave as the result only 380 candles of light, as proceeding from one horse-power, while the general claim for at least two of the dynamo-electric machines is nearly five times this value (200 becs Carcel being claimed for the Lontin machine and lamps as given by one horse-power. As each Carcel burner equals 9.6 candles of the English standard, it follows that this claim is for 1920 candles per horse-power). The Franklin Institute experiments also gave 31 to 38 per centum of the whole power as all that appeared in the resistance of the electric arc, which evolves the light, the remainder being expended in friction of machine, and in the circuit, being dissipated as heat to the air probably.

=

"Taking the value of 380 candles as proceeding from one horsepower, we have, the force corresponding to one horse-power equals. 1,980,000 pounds raised one foot high per hour-a unit of heat 772 foot pounds one horse-power =2565 units of heat per hour, and 2565 ÷ 380 gives 62 units of heat per candle.

"From which it appears that the theoretic expenditure of heat (or force) in producing gaslight is to that in producing electric light as 230 to 61, or as 34 to 1, while the theoretic expenditure of heat in producing gaslight is to that of the electric light in the arc alone, is very nearly as 100 to 1. All these are based on the Franklin Institute values; if the figures of M. Lontin are accepted the ratios become five times more favorable for electric lighting.

"But the heat which can be evolved by the burning of coal gas does not represent the heat which appertains to the coal that would have produced the same coal gas. Each pound of gas coal which has undergone the process of destructive distillation will have produced very nearly, on the average, 4 cubic feet of illuminating gas, and there will remain, as a residue of the process of gas making, about 0.85 pound of gas coke, and some 0·04 or 0·05 pound of gas tar, both of which substances are combustible. The quantity of coke, however, is reduced by the necessary use of coke in obtaining the heat for distillation until only 0.55 to 0.60 pound of unconsumed coke remains for each pound of coal originally charged in the retorts. Again, much of this coke is in the condition of dust or very fine breeze, so that practi

cally only about 0·35 to 0·40 of gas coke is derived in merchantable or useful shape for each pound of coal originally treated; while the coal tar, although it possesses the constituents of a fuel, is not easily burned as such. Ordinary gas coal can be taken to have the value of 15,000 units of heat to each pound, and if it is supposed that from this quantity of heat there be deducted the heat value of, say, 0·6 pound of coke, taken at 13,000 units to the pound = 7800 units, together with the heat value of 0.05 pound of gas tar, taken at 20,000 units to the pound = 1000 units (total deduction 8800), we have 6200 units as the value of the fuel which yielded 4 cubic feet of gas. And, as before, taking each cubic foot of gas as evolving three candles of light, it follows that 517 units of heat will have been expended for each candle of light emitted, and this value should be taken as the expenditure of heat in lieu of the 230 units which the gas would produce after it is manufactured.

"This statement compared with the theoretic heat of electric lighting, as before estimated at 62 units per candle power, gives the heat in producing gaslight to that of producing electric light as 517 to 63, or as 76-6 to 1, while the theoretic expenditure of heat in producing gaslight is to that of the electric light in the arc itself as 230 to 1 with the multiplication of these ratios by five to follow, if M. Lontin's development of light per horse-power is taken in place of that of the Franklin Institute.

"Unfortunately for this statement, in a practical point of view, the production of the electric light by the dynamo-electric machines involves the employment of motive power in the form of the steam engine (I omit to consider the gas engine as the source of motive power, thinking that its adaptation to general use should precede a discussion of its economy in this special application). And in none of the utilizations of natural forces is quite so great waste of original force as in the combustion of fuel in the steam engine. The best of engines can scarcely be claimed to give out a horse-power with less than 2 pounds of coal per hour, while the average of good engines of such size as would be at all available for power to run dynamo-electric machines of ordinary size, cannot be taken to average less than 6 pounds per per horse-power per hour. The former of these weights of coal represents 30,000 units of heat per hour to produce 2563 units of effect, or only 8.55 per centum of the expended fuel, and the latter only 2·85. per centum (one thirty-fifth) of the same.

66

'Taking this latter value as the practical one, the 63 units of heat per candle, which were the theoretic result of the Franklin Institute experiments, becomes 235 units of heat, in fact, needful to produce each candle. Our previous figures have given 230 units of heat as what would proceed from the best consumption of coal gas capable of producing one candle's light, and consequently the comparison of the heat practically demanded to produce electric light by the dynamo-electric machine actuated by the ordinary steam engine with the theoretic heat from coal gas is about 2 per centum against the electric system.

"But the previous figures also give 517 units of heat as what is expended in the making of coal gas for each candle's light, and this value compared with that for the dynamo-electric light with its motive (steam) power is 2-2 times less favorable for the system of gas lighting.

There is another way to look at the practical question. One pound of coal will produce 4 cubic feet of illuminating gas, each foot of which will have three candles' power; then omitting the other products of coal, 0·083 pound of coal represents the fuel expended in giving the light of one candle in the same time by coal gaslighting. Following the previous assumption that 6 pounds of coal will under the boiler of a steam engine evolve one horse-power, and taking the Franklin Institute value of 380 candles per horse-power from the dynamo-electric machine, then 0·0158 pound of coal represents the fuel in this case. This gives the practical relation of expenditure of coal (as fuel) in producing gaslight to that of the electric light under discussion 5.25 to 1. This last view of the case is practically not an unfair one, for while the coke and the coal tar are not absolutely waste products in process of gas making, they are by no means profitable ones in themselves, and it is questionable if they pay much more than the cost of handling and disposing of them, especially at places where coal for fuel is relatively cheap.

=

"It must be noticed all through this paper that the assumption has been for the combustion of fuel by an ordinary steam engine and boiler at 6 pounds of coal per horse power, while a moderately good engine with boiler ought and will give a horse-power for 3 pounds of fuel, and the best engines will only require under 2 pounds for the same result, so that all the practical ratios are two to three times less favorable towards the electric system than they might be.

"It remains, therefore, that the most unfavorable statement of the

relative expenditures of heat and fuel practically, of the electric system by the dynamo-electric machines and steam engine, as compared to that of coal gas for equal quantities of light is, at present, 1 to 2-2; while in the processes there will have been burned as fuel one pound of coal under a steam boiler, against 5 pounds of coal treated in the retort for the manufacture of coal gas. Whether the cost of manipulation of the fires or the retorts, the wear and tear of machinery and engines employed in either processes, the interest upon the prime cost of apparatus, from the fire-places to the points for illumination may be in the same ratio as either the heat demanded or the coal used, is a question for further consideration and for future development.

"Coal gaslighting would seem to have attained nearly to its ultimate development and improvement. Minor advantages of cost of manufacture, and possibly in the direction of quantity of light evolved by a given consumption, can be confidently looked for. The public have been and are habituated to all the defects of the system, and from the beginning to the end they are grave and essential. On the other hand the electric light has the promise of the greatest improvement in methods of production, in its adaptation to defined wants, and in its relative cost, so that the unfavorable view of it which I have here presented should not be taken, and it is far within the limits of probability that the best statement I have presented underrates its possibilities.

"It may be proper to advert here to the low result in candle-power for the electric light found by the Franklin Institute experiments. The power of light emitted by the candle or the gas-burner usually tested by it, is so small that the radiant light from the sides of a dark chamber may be neglected. The measurement of light was taken in all instances from the rays of a standard candle as emitted on the horizontal plane through the centre of the flame. It was found first, that this radiant light upon the walls of a dark box from the electric light was very great, and the means shown in the report of the committee (where it will be seen that only a pencil of light of not over a single degree of aperture was directed into the box or tube carrying the photometer) were found effectual to give a highly satisfactory comparison of the greater with the lesser source of light under investigation. Next it was found in the carbon experiments that there was an apparent unsteadiness of light of great amount emitted from the carbon. As the experiments proceeded it became evident that this unsteadiness was