Efficiency of Incandescent Lamps. 477 



watt with the two systems of supply, but also the life of the 

 lamp, since the cost of lamp renewals may be as important a 

 question for the consumer as the bill for electric power. 

 Unfortunately, however, there does not exist, as far as we 

 are aware, any accurate information as to the life of an in- 

 candescent lamp with various alternating P.Ds.; indeed the 

 data at our disposal for the life of a lamp with various non- 

 alternating P.Ds. is at most meagre. And if any Member of 

 the Society can supply us with information regarding the 

 life of some fixed type of incandescent lamp either with 

 different direct P.Ds. or with different alternating P.Ds. we 

 shall be grateful for the information, as it will furnish us 

 with a further opportunity of using the method described in 

 our paper read before this Society for deciding " On the 

 Most Economical Potential Difference to Employ with In- 

 candescent Lamps." * 



For the present, therefore, we shall confine ourselves solely 

 to the question of efficiency, and as it is known that the 

 efficiency of an incandescent lamp increases with the current 

 passing through the lamp, it is clear that to obtain an accu- 

 rate comparison of the efficiencies with direct and alternating 

 currents, we must employ exactly the same current in both 

 cases, or rather the same mean square of the current, for this 

 will be most likely to develop the same rate of production of 

 heat in the lamp, since this rate is proportional to the re- 

 sistance into the mean square of the current, whether the 

 current be direct or alternating, and whether or not there be 

 self-induction in the circuit in which the heat is developed. 

 We say will be most likely to develop the same rate of pro- 

 duction of heat, since we must not assume without proof that 

 for the same mean square of current the resistance of a 

 carbon filament is the same whether the cm-rent be direct or 

 alternating. 



Some writers have stated that the current, as measured by 

 an electrodynamometer, required to be sent through an in- 

 candescent lamp when emitting a definite amount of light, 

 had a different value when the current was an alternating one 

 from what it had when the current was direct. This dif- 

 ference might be due either to a defect in the dynamometer 

 measurement or to some variation in the light standard. If 

 the wire with which a dynamometer is wound be thick, then 

 the current-density may be far from uniform when the current 

 is alternating ; and on this account, as observed by Captain 

 Cardew some years ago, a dynamometer might give different 

 readings for different rates of alternations of the current, 

 * Phil. Mag. April 1885. 



