Tests of Glow- Lamps. 417 



costing 9d. per Board of Trade unit, no economy will be 

 gained by discarding the lamps before the filaments break. 



This important result is of course due to the fact, already 

 pointed out, that there is no marked decrease in the candle- 

 power of the lamps during their lives. 



Owing to the extent to which the individual lamps varied 

 among themselves, it is difficult to say which of the four 

 pressures at which we tested lamps would be the most 

 economical to use. The three lamps tested at 101 volts seem 

 to have been quite abnormal, their efficiency being very low, 

 and, as a consequence, their lives very long. 



With a Board of Trade unit costing 4-^., the lamps tested 

 at 100 volts gave the cheapest light, and as they do not seem 

 to have been above tho average in quality this result may 

 probably be safely taken as correct. 



With energy costing ikl. per Board of Trade unit (fig. 13) 

 there does not seem much to choose between the total cost of 

 producing light with the three pressures of 100, 102, and 104 

 volts, excepting that the use of 104 volts would introduce an 

 extra expense arising from the labour of frequently replacing 

 broken lamps. 



The two conclusions, then, to which we are led by a study 

 of these total-cost curves on figs. 12 and 13 are : — ■ 



1. The modern Edison-Swan 8 C.P. 100-volt lamps such 



as we tested should not be discarded until the filaments 

 break. 



2. No marked economy can be gained by over-running this 



type of lamp. 



Addition made January 25, 1895. 



The question arises — what is the cause of the marked rise 

 in candle-power which occurred with all the lamps tested by 

 us during 1893 and 1894 ? In the early summer of 1894, 

 when our tests were approaching completion, Mr. Howell, in 

 contributing to a discussion at the American Institute of 

 Electrical Engineers, stated that such a rise in candle-power 

 was due to the vacuum of the lamp improving during the 

 early part of its life. Therefore, after we had stopped the 

 main investigation, we made some experiments to test the 

 validity of Mr. Howell's explanation. 



We applied the induction-coil vacuum-test to some dozen 

 Edison- Swan lamps when quite new, and again after they 

 had been run for about 80 hours at their normal pressure 

 of 100 volts. In every case we found that the vacuum had 



