Line Spectrum emitted by Iron Vapour. 285 



Amount of penetration 



of rod into heated Ionization 



portion of tube. Current. 



inch ampere 



0-5 „ 0-1 „ 



1-0 „ 0-6 „ 



1-5 „ 1-0 „ 



2-0 „ 1-5 „ 



•2-5 „ 2-0 „ 



3-0 „ 2-3 „ 



3-5 „ 2-6 „ 



4-0 „ 2-8 „ 



From these results it will be seen that an appreciable 

 current is obtained when the rod penetrates only \ inch. 

 The current then increases rapidly as the rod penetrates 

 farther into the; tube, partly on account of the higher tempe- 

 rature with consequent higher degree of ionization prevailing- 

 near the middle of the tube, and partly also on account of 

 the increasing area of active rod surface and the rising 

 potential difference between the extremity of the rod and the 

 opposite furnace-wall. After the rod has passed 2*5 inches 

 into the tube the rate of increase of the ionization current 

 diminishes, no doubt because the rod now enters cooler 

 regions of- the furnace, in which the intensity of ionization 

 declines again. It should of course always be borne in mind 

 that the ionization current does not pass only between the 

 extremity of the rod and the wall, but that the flow of elec- 

 tricity takes place, more or less, all along that portion of the 

 rod which is within the heated zone of the furnace-tube. 



A further set of experiments was carried out at higher 

 temperatures, the extremity of the rod being at the middle 

 of the furnace, namely 2*0 inches from the end of the heated 

 portion of the furnace-tube. At the highest temperature 

 observations were made both with and without iron vapour 

 in the tube. The following values were obtained : — 



Furnace Ionization 



Tempeiature. Current 



2400° C 3-0 ampere "| Without 



2700 „ 3'6 „ J iron vapour. 



2700 „ 4*4 „ With iron vapour. 



The last result shows that a heavy ionization current 

 passes even when iron vapour is present, and I suppose that 

 the higher value obtained is due to part of the current 

 being now carried by iron vapour. 



