Cady and Arnold — Electric Arc. 



397 



To get rid of the oxidation, an apparatus was constructed 

 for forming in nitrogen arcs with anodes of various metals. 

 This consisted of a bell-jar about 27xl2 cm cemented onto a 

 brass base. The substance serving as anode, in the form of a 

 small pellet, was laid in a cavity in the top of a massive brass 

 block, in order that the heat might be conducted away as rap- 

 idly as possible. The upper (negative) electrode was a carbon 

 8 mm in diameter held by a small brass rod, which passed up 

 into a glass tube. The latter was cemented with ceiling wax 

 into the neck of the bell-jar. At the upper end of the brass 



VOLT 



























f 























V 





















300 



\ 

 \ 



\ 





















\ 



\ 



\ 



















200 





\ 

 \ 





















\ 



\ 



\ 



















100 





\ 

 \ 





















' ) 

















F 





c 









G 





















H 





I 2 AMP. 



Fig. 4. Points A to F, glow and arc with silver anode. Points G and 

 H, arc with lead anode. 



rod was an iron button. By means of a strong electro-magnet 

 whose poles approached close to the tube on each side, the 

 weight of the carbon was sustained and it could be raised and 

 lowered at will. 



§23. It was thought that at low pressure in nitrogen the 

 critical point might be observed more readily than in free air. 

 This we found to be the case. With a lead anode, in nitrogen 

 up to 12 cm pressure, only the glow discharge was observed. 

 The cathode glow formed a mantle covering the negative ter- 

 minal to an extent depending on current and gas pressure. 

 The dark space extended from this to the positive column, 



