1882.] on Electric Ligliting hij Incandescence. 39 



know by actual experiment that continuous heating to a fairly high 

 degree of incandescence during 1200 hours does not destroy a well- 

 made lamp. What the utmost limit of a lamp's life may be, we 

 really do not know. Probably it will be an ever-increasing span ; as, 

 wdth increasing experience, processes of manufacture are sure to 

 become more and more perfect. Taking it, therefore, as fully esta- 

 blished that a cheap and durable lamp can now be made, the further 

 question is as to the cost of the means of its illumination. 



This question in its simplest form is that of the more or less 

 economical use of coal; for coal is the principal raw material alike in 

 the production of gas and of electric light. In the one case, the coal 

 is consumed in producing gas which is burnt ; in the other in pro- 

 ducing motive power, and, by its means, electricity. 



The cost of producing light by means of electric incandescence 

 may be compared with the cost of producing gas light in this way, — 

 2 cwt. of coal produces 1000 cubic feet of gas, and this quantity of 

 gas, of the quality called fifteen-candle gas, will produce 3000 candle- 

 light for one hour. But besides the product of gas, the coal yields 

 certain bye products of almost equal value. I will, therefore, take it 

 that we have, in effect, 1000 feet of gas from one cwt. of coal instead 

 of from two, as is actually the case. 



And now, as regards the production of electricity. One cwt. of 

 coal — that is the same measure in point of vcdue as gives 1000 feet of 

 gas — will give 50 horse-power for one hour. Repeated and reliable 

 experiments show that we can obtain through the medium of incan- 

 descent lamps at least 200 candle-light per horse-power per hour. 

 But as there is waste in the conversion of motive power into electri- 

 city, and also in the conducting wires, let us make a liberal deduction 

 of 25 per cent., and take only 150 candle-light as the nett available 

 product of 1 horse-power; then for 50 horse-power (the product 

 of 1 cwt. of coal), we have 7500 candle-light, as against 3000 candle- 

 light from an equivalent vcdue of gas. That is to say, two and a half 

 times more light. 



There still remains an allowance to be made to cover the cost of 

 the renewal of lamps. There is a parallel expense in connection 

 with gas lighting in the cost of the renewal of gas-burners, gas 

 globes, gas chimnies, &c. I cannot say that I think these charges 

 against gas lighting will equal the corresponding charges against 

 electric lighting, unless we import into the account — as I think it 

 right to do — the consideration that, without a good deal of expense 

 be incurred in the renewal of burners, and unless minute attention 

 be given, far beyond what is actually given, to all the conditions 

 under which the gas is burned, nothing like the full light product 

 which I have allowed to be obtainable from the burning of 1000 cubic 

 feet of gas, will be obtained, and, as a matter of fact, is not commonly 

 obtained, especially in domestic lighting. Taking this into account, 

 and considering what would have to be done to obtain the full yield 

 of light from gas and that if it be not done, then the estimate I have 



