Tests of Glow -Lamps. 397 



In calculating the figures from which the bottom curve is 

 plotted, we have taken the cost of a new lamp as one shilling, 

 and the price of a Board of Trade unit as 4J<#., and, as seen, 

 the ordinate of this curve reaches a minimum at about 600 

 hours, when the cost per hour per candle has dropped to 

 0*0235 pence. Beyond this point the curve begins to ascend ; 

 that is, when a new lamp costs one shilling, and a kilowatt- 

 hour 4Jc?., and lamps whose life-histories are truly represented 

 by the curves M in fig. 3 are employed, the cost of obtaining 

 light is least if the lamps are used for 600 hours only, because 

 after that time their diminished quality more than overbalances 

 the cost of renewing. 



A change in the price of lamps does not much affect the 

 economical life, as is shown on fig. 5 by the middle curve, 

 which we have calculated for the same price of power but 

 on the assumption that a new lamp costs two shillings. The 

 ordinate of this curve has a minimum value at 650 hours 

 instead of at 600 hours, when a new lamp was supposed to 

 cost one shilling only. 



The higher the price paid for energy the more important 

 does this question of economical life become, and the sooner 

 is it necessary to discard lamps, because the cost of renewals 

 bears a less proportion to the total cost. This is illustrated 

 in fig. 5 by the top curve, to obtain which we have assumed 

 that the cost of energy was 9d. a unit, and the price of a new 

 lamp one shilling. Now the minimum point falls to 430 

 hours instead of being at 600, at which it stood when the 

 price of the Board of Trade unit was taken as k\d. 



When cost curves, like those in fig. 5, are drawn for a 

 worse type of lamp, the minimum point becomes more sharply 

 defined and is reached earlier. It might be urged, however, 

 that all such curves merely indicate for how many hours 

 lamps should be used in order that the price paid for the 

 light may be a minimum, but that, if at the end of that time 

 the light given out by the lamps is sufficient for the purpose 

 for which they are intended, surely it would be folly to 

 discard them. The answer to this is, that the lamps employed 

 were of too high candle-power for the necessary illumination, 

 and what should be done is to throw away the nominally 

 higher candle-power lamps that have deteriorated and replace 

 them with new lower candle-power lamps. 



When, therefore, the special investigation which forms the 

 subject of this paper was commenced at the end of 1892, 

 there was good reason for expecting, first, that with any type 



