Tests of Glow-Lamps. 411 



Although, as already stated, the number of lamps that could 

 be supplied with power every night at the Central Technical 

 College was limited to nine, and although, therefore, the total 

 number dealt with in the investigation was small, the curves 

 on figs. 10 and 10a, and the results given in the preceding 

 table, are quite sufficient to enable us to arrive at the following 

 results : — 



(1) When a group of Edison-Swan lamps marked 100-8 



are run at 100 volts, and each lamp, as its filament 

 breaks, is replaced by a new Edison-Swan 100-8 lamp, 

 it may be expected that the light given out by the 

 group will never be as small as it was at the beginning, 

 when all the lamps in the group were new. This im- 

 portant result is brought about by the deterioration 

 of the lamps with long-lived filaments being com- 

 pensated by the great rise in the light given out by 

 each new lamp when put in place of one whose fila- 

 ment has broken. 



(2) An Edison-Swan lamp marked 100-8 when run at 



100 volts will give an average illumination during its 

 whole life of about 10 candles, and will absorb an 

 average power of about 4*3 watts per candle, so that 

 such a lamp must be regarded as a 43-watt lamp, and 

 not a 30-watt lamp as is not unfrequently stated, this 

 difference in power being about 43 per cent. 



(3) An Edison-Swan lamp marked 100-8 may, when 

 run at 100 volts, emit during a large portion of its life 

 a light of as much as 11*7 candles, and absorb a power 

 of about 44*6 watts, which is about 44 per cent, greater 

 than the nominal 30 watts. 



These last two facts are of great importance to dynamo- 

 constructors, when specifying, as is frequently done, the 

 number of lamps of a given type to which a given dynamo 

 can supply current to without becoming too hot. For if, in 

 making such a calculation, it be assumed that an Edison- 

 Swan 8-candle-power lamp absorbs the nominal 30 watts, 

 that is 3*75 watts a candle instead of the 44 # 6 watts which 

 our tests show that such a lamp actually absorbs during a long 

 portion of its life, the current will be 44 per cent, greater, 

 and the rate of heating of the dynamo 108 per cent, greater, 

 than was anticipated. 



(4) Groups of new Edison-Swan lamps marked 100-8, if 

 selected so as to give the same light at 100 volts, will, 

 when new, emit a light which is roughly proportional 

 to the seventh power of the pressure applied to them. 

 But after a run of 100 hours, this rule connecting light 

 and pressure may, or may not, hold. 



