Tests of Glow-Lamps. 421 



any Edison-Swan lamps marked " 100 E.F. 8 ;" since batches 

 of lamps so marked, and therefore nominally the same, really 

 require very different potential differences to be employed to 

 obtain maximum economy with a given price of lamp and of 

 a kilowatt-hour. 



It may, however, be concluded, from the curves shown in 

 figs. 12 and 13, that the most economical potential difference 

 to employ with any Edison- Swan lamps marked " 100 E.F. 8," 

 such as have been obtainable in the open market since 1892, 

 is the potential difference that will cause the lamp during a 

 large portion of its life to have an efficiency of about 0*25 

 candle per watt, corresponding with 4 watts per candle. 



The value of this potential difference for any batch of these 

 lamps may be determined approximately in the following 

 way. From our tests, already referred to, of eleven Edison- 

 Swan lamps, marked " 100 E.F. 8," which had been used in 

 a bouse for some 200 or 300 hours, we found that the relation 

 between candle-power and pressure was given by 



Candle-power = ax (volts) 5 " 91 , 



where a is a constant. Also we found that the relation 

 between candle-power and watts for these lamps was given by 



Candle-power = bx (watts) 2 ' 9 , 



where h is a constant. Therefore 



Volts = c x (watts per candle) -0 ' 258 , 



where c is a constant. 



Hence if one has a batch of Edison-Swan lamps marked 

 te 100 E.F. 8/' whose efficiency is, say, 0*222 candle per watt, 

 corresponding with 4*5 watts per candle, when a pressure of 

 100 volts is applied to them, it would be probably most 

 economical to run them at about 103 volts, since this is the 

 pressure which the last equation tells us will be necessary 

 to raise the efficiency of such lamps to 0*25 candle per watt. 



From what precedes, it follows that, since the average 

 efficiency of the 100 E.F. 8 Edison-Swan appears to be less 

 than 0*25 candle per watt, the opinion expressed by one of 

 us in an article entitled " New Lamps for Old," published in 

 the ' Electrician ' for September 29, 1893, was correct, viz. 

 that it would be economical to overrun Edison-Swan lamps 

 by applying a pressure about 3 per cent, higher than the 

 marked pressure. And that the reason why this conclusion 

 was apparently negatived by the results of the tests which we 

 carried out during the winter of 1893 and a large portion of 

 1894, was because the various groups of lamps which we ran 

 Phil. Mag. S. 5. Vol. 39. No. 240. May 1895. 2 F 



