ELECTRIC TRANSMISSION IN VACUA. 153 



Hence electric action within this space is neutralized ; 

 producing the dark shadow c d shown on the broad 

 end ; while the rest of the tube is illuminated. 



When the screen is thrown down a luminous cross 

 takes the place of the dark shadow: but this higher 

 illumination soon fades, since electric action on this 

 space is now the same as on the rest of the tube. 



If the tube be used again, after a period of rest, the 

 shadow can be reproduced ; but is never so strong as at 

 first. This proves that the glass has been subjected to 

 an electric strain, which has permanently lessened its 

 insulating power. 



The illumination of the glass is due to its resistance ; 

 just as the bright spark is due to the resistance of air 

 at the ordinary density, and the faint glow, to the 

 reduced resistance in vacuum. Hence, when electric 

 action begins, after the screen is thrown down, the 

 resistance being greater on the spot which was pro- 

 tected by the screen, we have the bright cross where 

 the dark one was : but when the electric strain has so 

 affected the relations of the molecules to each other, as 

 to lessen the resistance, this first bright glow ceases, and 

 the illumination is the same as in the rest of the tube. 



This action on the glass, as shown in Figs. 47 and 48, 

 is accompanied with heat as well as light ; the- tube 

 shown in Fig. 47 becoming intensely hot, at those 

 points where the greatest electric energy is concen- 

 trated. 



Fig. 49 represents a tube constructed to show this 

 heating effect in a very striking manner. Its upper 

 part is enlarged into a globular form : and, at the bot- 

 tom, is the concave negative electrode, of aluminium, 

 already described ; which is so placed that it brings 



