SELECTION OF AN ILLUMINANT 101 



be made in the following order : Automatic arc, hand-feed 

 arc, Nernst lamp. 



For alternating current : Nernst lamp, which iJiajr be 

 supplemented when a more powerful light ,-> is' requireH 

 by a hand-feed arc. 



In the absence of the electric current : Limelight^ in-' 

 candescent gas or vapour lamps, oil, acetylene ; the oil 

 lamp taking first place for strictly critical work not 

 requiring a very intense light. 



At present mercury vapour lamps hardly take the 

 place to which their good features would entitle them, 

 on account of their fragility and a lower intensity than 

 that of arc or limelight, but improvements may at some 

 future time place them quite in the forefront of artificial 

 illuminants. 



Approximate power of Illuminants. The following 

 table may be of service in the selection of an illuminant 

 as showing the candle-power given by some of the most 

 common. The figures are only approximate, and vary 

 in each case according to the particular conditions, but 

 they give a rough idea of the comparative exposures 

 necessary. 



Candle-power. 



Flat-wick oil lamp . . . . 15 



Incandescent gas . . . 35-60 



Acetylene . 40 



Spirit and petrol vapour lamps . . 50-300 



1 -ampere Nernst, 200 volt . . 200 



Blow-through jet .... 400 



Injector jet .... 1000 



Mixture jet . . . 1000-2000 



4-ampere hand-feed arc . . . 300-500 



5-ampere automatic arc . . . 1000 



20-ampere automatic arc . . . 2500 



35-ampere automatic arc . . . 4000 



7 5-ampere automatic arc . . . 10,000 



