334 ELEMENTARY LESSONS ON [CHAP. vn. 



circuit (Art. 404), which possesses a high electromotive- 

 force, can leap the short distance, and in doing so 

 volatilises a small quantity of carbon between the points. 

 Carbon vapour being a partial conductor allows the 

 current to continue to flow across the gap, provided it be 

 not too wide ; but as the carbon vapour has a very high 



Fig. 137. 



resistance it becomes intensely heated by the passage 

 of the current, and the carbon points also grow hot. 

 Since, however, solid matter is a better radiator than 

 gaseous matter, the carbon points emit far more light 



