4 SCIENCE PROGRESS 



accustomed, however, to switching on the electric light and 

 reckoning the convenience of the lamps in this respect one of their 

 chief advantages, the public naturally asked for an automatic 

 means of starting the light. This is ingeniously provided by sur- 

 rounding the filament with an open wound porcelain spiral itself 

 closely wound with fine platinum wire : on switching the lamp 

 into circuit, the current at first flows through this " heater " spiral, 

 raising it to a bright red heat ; the filament being now heated by 

 radiation and convection and, in twenty or thirty seconds is so hot 

 that it becomes conducting, and the current then flows through 

 it. In series with the filament is the coil of a small electro- 

 magnet actuating an armature which breaks the heater circuit 

 as soon as the current is flowing through the filament. Addi- 

 tional complication is introduced, however, by the fact that, like 

 all pyro-electric conductors, the filament falls in resistance the 

 hotter it gets. In consequence the current is unstable, a slight 

 rise in potential difference between the ends of the filament 

 producing an indefinite increase in current which continues to 

 rise until the filament fuses. To check the rise it is necessary 

 to run the filament in series with a resistance ; for this purpose 

 a most ingenious form of resistance has been invented, which 

 consists of a fine iron wire in a vacuum, like a small glow-lamp 

 with an iron wire filament. This steadying resistance has a 

 characteristic markedly opposite to that of the filament itself, 

 the resistance of the iron wire at a dull red heat increasing very 

 rapidly with a slight increase of temperature due to a slight 

 increase of current. In combination with this special resistance 

 the Nernst filament is a perfectly stable conductor. 



These two complications had a very unfavourable effect on 

 the success of the lamp. Of a candle-power range which made 

 its use possible in competition with the carbon filament lamp 

 in cases in which its much higher efficiency must have ensured 

 success, the steadying resistance, the heater and the magnetic 

 cut-out converted the Nernst lamp into a sort of miniature arc 

 lamp. The small user was chary of adopting so complicated a 

 mechanism having so many parts liable to get out of order and 

 calling for almost expert knowledge for their maintenance ; the 

 large user preferred the more efficient arc lamp, the mechanism 

 of the individual lamp being scarcely more complicated than 

 that of the Nernst lamp and much less complicated for equiva- 

 lent candle power, as one arc lamp is equivalent to three or 



