Induced Alternating Current Discharge. 67 



or 25 cm without extinguishing the flame, presenting a striking 

 and beautiful spectacle. 



It may be of interest here, in view of the peculiar character 

 of the discharge, to give the results of some experiments which 

 were made a few years ago in this laboratory to determine the 

 time and mode of its formation. A camera was arranged so 

 that the discharge, just as it was forming, could be photo- 

 graphed upon a rapidly moving plate. At the same time the 

 image of a spark from a tuning fork whose point dipped in mer- 

 cury was adjusted so as to fall upon the plate side by side with 

 that of the discharge. The photographs show that the spark 

 passes directly across at first. As each spark has a tendency 

 to follow its predecessor and the air becomes heated causing an 

 upward current, the path of the discharge rises upward little 

 by little until it reaches its permanent condition. From the 

 period of the tuning fork it was calculated that about one- 

 tenth of a second was required for this. 



In the Chemical Xews* Crookes has an article on this flame 

 in which he states that it consists chiefly of nitrogen burning 

 with the formation of nitrous and nitric acids, and that it shows 

 no lines, but that the spectrum is faint and continuous. It is 

 more probable though that the conditions here are similar to 

 those in a tube from which a portion of the air has been 

 exhausted. Owing to the heat, the air within the aureola 

 becomes rarified and partially conducting. The luminosity is 

 caused by the air being heated and electrified. Stratifications 

 can be seen and photographs show them distinctly somewhat 

 as they are shown in a vacuum tube. 



An examination of the spectrum with terminals of several 

 different metals, made by means of an ordinary prism spectro- 

 scope, shows in all cases, and as the most prominent feature, a 

 continuous background which is very bright in the red, yellow, 

 and green, but gradually decreases in intensity until it fades 

 away in the violet. The yellow sodium lines are always pres- 

 ent, due to particles in the air or impurities of the electrodes. 

 Generally a few of the stronger lines, which are due to the metal 

 of the electrodes and are ordinarily seen in its flame spectrum, 

 appear and others which come out only occasionally. With zinc 

 terminals and also with those of aluminum, the flame is yellow- 

 ish in color but has a peculiar bluish-green core which is very 

 intense, extending in streaks throughout the flame. The dis- 

 charge from electrodes of soft iron which are small in diameter 

 presents a beautiful spectacle, inasmuch as the iron, because 

 of its large resistance, becomes highly incandescent, and bursts 

 into a brilliant combustion, throwing off luminous particles in 

 all directions. A banded structure from about wave length 



* June 11, 1892. 



