THE PROGRESS OF THE ELECTRIC LIGHT 3 



extra cost of producing the light is so considerable that it 

 more than balances the cost of a new lamp. The precise 

 temperature (or the watts per candle l ) at which a lamp should 

 be used to give the most economical result depends on the 

 price at which the lamp can be bought and the cost of energy ; 

 it can be calculated easily when these particulars are known. 

 But commercial usage strikes a rough average of scientific 

 calculations and it is found in practice that for general purposes 

 carbon filament lamps should consume between 3 and 4 watts 

 per candle when new : under such circumstances they will have 

 a life of about 1,000 hours and an average energy consumption 

 throughout their life at the rate of from 4 to 4*5 watts per candle. 

 Such then was the best which the electrical engineer could 

 produce up till 1898, when altogether fresh possibilities were 

 made obvious by the invention of the Nernst lamp ; this at one 

 stroke practically halved the power consumption per candle. 

 The Nernst lamp was a radical improvement on the carbon 

 filament, more radical indeed in many respects than the metallic 

 filament lamps which have succeeded and eclipsed it. Instead 

 of a filament of a material electrically conductive at all tem- 

 peratures, Nernst used one made of a mixture of oxides which 

 would ordinarily be classed as non-conducting but which at 

 moderately high temperatures became conducting. This 

 necessitated the preliminary heating of the filament ; but once 

 its temperature had been raised to about a red heat the heating 

 effect of the current itself sufficed to maintain the filament in 

 a conducting state. The filament is not affected by the air, so 

 is not enclosed in an exhausted bulb. The Nernst lamp can 

 be started easily by heating the filament with a match ; being 



1 The candle power of a light source is the measure of the intensity with which 

 the source radiates light in a given direction compared with the intensity of a 

 standard source — the candle. The value of the standard candle varies slightly in 

 different countries, no international standard having been as yet generally adopted. 

 In this article the English standard candle is taken as unit. The watts consumed 

 by a lamp measure the electric power : one ampere of current flowing under a 

 pressure of one volt is equal to one watt : under such conditions in an hour one 

 watt-hour of energy is consumed. The efficiency of a given electric-light source 

 is therefore measured by the number of candles given per watt. It is, however, 

 general to compare lamps by the inverse function — the number of watts per 

 candle — and this is often wrongly spoken of as the efficiency, though it is really a 

 measure of the inefficiency. The actual temperature of the filament being hard 

 to determine, the watts per candle serve as a useful basis for comparison, being 

 of course lower the higher the temperature. 



