SOUECES OF ILLUMINATION 



89 



is fairly thick and straight, and the light is then of sufficient 

 intensity and of suitable area for work with high powers (Fig. 26). 

 As the filament will not conduct electricity until its tempera- 

 ture is raised, the preliminary heating may be effected by 

 holding it over a small Bunsen flame, or a separate electrical 



FIG. 26. Nernst Electric ' Luna ' Burner. 



heater is mounted close to it. On allowing current to pass, 

 the temperature of this heater is at once raised. It 

 is in close proximity to the illuminating filament, and con- 

 sequently the temperature of the latter is raised also. When 

 the temperature of the filament reaches from 600 to 800 C. its 

 conductivity is increased sufficiently to allow current to pass 

 through it. By an automatic arrangement the current is then 

 cut off from the heater and passes only through the illuminating 

 filament. The lamp does not light immediately the current 



