2 SCIENCE PROGRESS 



of the flame arc, progress at the present time is certainly at 

 a very much less rapid rate than it was, say, five years ago. 

 It is usually so with inventions of this kind. At first there is 

 a period during which the invention is being worked out in the 

 laboratory (generally, it may be noted with regret, the labora- 

 tory of some German manufacturer), when the public naturally 

 knows nothing of what is going on : then comes the announce- 

 ment of the invention and its first display in a more or less 

 crude commercial form, this being followed by a time of rapid 

 progress when almost each week brings forth rumours of fresh 

 discoveries far outstripping those already published. For a 

 time, even the professional public scarcely knows which way 

 to turn or whether the brilliant achievement of to-day may not 

 be eclipsed by the still more brilliant achievement of to-morrow ; 

 but before long the goats are separated from the sheep, 

 principles and methods of manufacture which are really on a 

 sound basis begin to emerge clearly and the new position is 

 seen in its true light ; extravagant hopes fade and with them 

 fade extravagant fears ; in their place arises a genuine con- 

 viction of the true scope of the new invention and an earnest 

 endeavour to gather its fruits to the full. Such a period has 

 been reached in the case of the two new types of electric lamp ; 

 the present is therefore a suitable opportunity for a general 

 survey of the position. 



The glow-lamp or incandescent electric lamp in which the 

 filament is of carbon was invented in 1878 and during twenty 

 years was the only type of electric lamp suitable for interior 

 lighting. During that period it was brought to a high state of 

 perfection and was made suitable for a large range of voltage 

 and candle power. Carbon would indeed seem to be an ideal 

 material for the filament of an electric lamp, as it remains solid 

 at the highest temperature that can be produced : unfortunately, 

 however, the carbon filament in a lamp undergoes changes 

 which prevent it from being used at too high a temperature. 

 The higher the temperature at which it is run the shorter its 

 "life," whether the life be counted as the actual life until the 

 filament fails or as the "economical life" until it is cheaper 

 to discard the lamp and substitute a new one : for such a 

 point usually arises because the lamp deteriorates in use, 

 giving gradually less light though consuming the same or 

 even more energy, so that there comes a time at which the 



