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L V III. The Efficiency of Incandescent Lamps with Direct and 

 Alternating Currents. By W. E. Ayeton and John 

 Pekky*. 



IT is now well understood that in order to economically 

 distribute power by means of electricity it is necessary 

 to employ a high potential-difference, or P.D., between the 

 mains and a small current flowing through them, while con- 

 siderations of safety require that the P.D. between the leads 

 in the houses shall not exceed 100 or 200 volts. Hence some 

 system of converting a large P.D. and small current into a 

 small P.D. and large current has to be employed ; and the 

 four systems of conversion that have hitherto been devised 

 consist in the employment of: — 1. Motor-Dynamos, 2. Accu- 

 mulators, 3. Alternating Current-Transformers, 4. Direct 

 Current-Transformers. 



Of these four methods the third is the one that is most 

 extensively utilized at the present time ; indeed it is the only 

 system of conversion that is at all extensively used, at any rate 

 in this country. But in view of some direct current system 

 of conversion also coming into common use, there arises a 

 question of considerable importance to the consumer, viz. Is 

 more light obtained with the same expenditure of power with 

 direct or with alternating currents ? And apart from con- 

 siderations of the electric distribution of power on a large 

 scale, the answer to this question is of importance in supply- 

 ing one factor towards the decision as to the relative ad- 

 vantages and disadvantages of direct and alternating currents 

 for detached installations. 



Where the electric energy supplied to a consumer has been 

 charged by meter, people have, as a rule, been content to 

 measure only the number of coulombs supplied, ignoring 

 altogether any variation in the volts ; but as electricity per se 

 apart from the P.D. is of no commercial value whatever, and 

 therefore is unlike water apart from pressure, it is clear that 

 in estimating the value of a supply of electric energy for 

 lighting purposes we must measure the watts and not 

 merely the current. The problem, therefore, that we have 

 attempted to solve is whether a Board of Trade unit (1000 

 watt-hours) is more valuable for lighting by incandescent 

 lamps when the current is direct or when the current is 

 alternating. 



For a complete solution of this problem we must ascertain 

 not merely the " efficiency of the lamp " or the candles per 



* Communicated by the Physical Society : read February 25, 1888. 



