1885.] when raised to High Incandescence. 229 



Experiment 8. — No. 7 Lamp. 



Filament. 



Shunt. 



Kemarks. 



Volts. 



00* 



<o 



Pi 



a 



Calculated 



resistance 



(hot). 



Calculated 

 E.M.F. 



Current in 

 milliamperes. 



Ohms. 







ohms. 











on 

 ou 





142-9 













*66 



136-4 











100 



0-77 



129-9 









Blue effect in globe visible. 



102 



0-79 



129 -1 









104 



0-82 



126-8 











106 



0*84 



126-2 





H 



<D 



a 





108 



86 



125 -6 





IS 

 '> 



o 

 'S 





110 



0-89 



123 -6 





<o 



o 



09 





112 



91 



123 1 







03 





114 



94 



121 -3 





| 







116 



0-98 



118-4 





B 



© 





118 



1-00 



118-0 





o 





120 



1-02 



117-7 











the filament is raised beyond a certain limit the interior of the glass 

 envelope is blackened by a layer of carbon, which has been deposited 

 by a Crookes bombardment effect. When the carbon filament is fixed 

 on copper electrodes, the interior of the glass sometimes becomes 

 coated with copper as well as with carbon, and the line between the 

 two is perfectly marked, showing that the bombardment takes place 

 in right lines. Experiment 1 shows how very high the electromotive 

 force can be carried, if it be steadily and rapidly increased, before the 

 filament is broken ; but practice shows that when once the blue effect 

 appears, destruction is only a question of time. Hence the blue effect 

 is an indication of the advent of disintegration, and a very useful 

 warning of danger ahead. 



13. Now it is clear that we have a combination of the phenomena 

 above described in the Edison effect. A continuous bridge of mole- 

 cules is formed between the junction of the carbon filament and the 

 metal plates inserted between its heels. They are found deposited 

 on the metal plate. A shunt is thus formed whose resistance is 

 measurable, and a definite current passes. This shunt is formed just 

 where the negative metallic connexion joins the heel of the carbon 

 filament, as we should expect from the investigations of Mr. Crookes. 

 The current is, however, weak and variable, and it is scarcely reliable 

 enough to be useful for practical purposes as was hoped by its 

 discoverer. When the critical point is reached the blue glow and 

 flame seem to pervade the whole bulb, and the arc-like effect, instead 



