Edison Effect in Glow Lamps. 



93 



If, however, the condenser is connected between the 

 negative electrode of the lamp and the metal middle plate of 

 the lamp, on insulating and testing it we find it has not the 

 slightest charge. 



It is very astonishing to see how instantly a condenser of 

 very large capacity is charged when one pole of the con- 

 denser is connected to the middle plate and the other to the 

 positive electrode of the lamp. 



§ 24. In considering the behaviour of the heated carbon 

 electrode and the cool metal plate in their respective powers 

 of discharging the positive or negative charge of the con- 

 denser, it seemed that the fundamental fact was the power of 

 the heated surface to discharge negative electricity out of 

 itself. Hence arose the question, how iar the observed facts 

 would be modified if the middle metal plate itself could be 

 also heated. One way by which this might have been done 

 would have been to have rendered this plate incandescent by 

 heating it by radiant heat concentrated by means of a powerful 

 mirror or lens. Some experiments tried in this way were 

 not satisfactory, and consequently a method was adopted in 

 which a middle plate of carbon could be rendered incandescent 

 electrically. 



Experiment 21. — A vacuum tube was provided with two 

 carbon conductors (see fig. 19), one the ordinary carbon 



Fig. 19. 



filament L of a 50 volt lamp, and the other the small carbon 

 S of a 4 volt lamp. The smaller carbon was sealed in the 

 usual way through the glass and placed so as to stand 

 symmetrically between the legs of the laiger carbon loop. 



The smaller carbon could be rendered incandescent by an 

 insulated battery of fifteen secondary cells, appropriate 



