on the Electric Arc. 135 



Fe-Cu. This was very little different from the last. The 

 iron electrode showed a surface of pure metal surrounded 

 by the black deposit which seems characteristic of iron 

 arcs in hydrogen. It was only slightly eaten away, but 

 in pits of some depth. Some slight traces of copper were 

 present on the end of the iron. The copper electrode 

 was covered with the black iron deposit. At one point 

 only, a small drop of pure metal like iron showed where 

 the arc had made contact. On removing the deposit 

 quite an amount of iron was found tb have been carried 

 over onto the copper. Thus we have, as with most of the 

 arcs, evidence of the double transfer of material. 



Cu-Al. The arc was a mixture of green and yellowish-red 

 colours, and was very unstable. There was no particular 

 deposit on the apparatus. The electrodes became rough- 

 ened up somewhat, and the aluminium was eaten into 

 considerably. The Al electrode was the upper cooled 

 terminal. 



Al-Cu. This arc was about as stable and bright as the last. 

 The electrodes were burned away considerably, and the 

 globe was much blackened. On filing the end of the 

 copper, the metal looked like brass in streaks. The alumi- 

 nium was covered with a slightly reddish-black deposit, 

 and its end was found, on filing, to be porous to a consider- 

 able depth. 



Fe-Al. This is a better arc than Al-Fe, being more in 

 one place than a general spark discharge over the sur- 

 faces o£ the electrodes. It continually hissed, as is general 

 with the metal arcs in hydrogen, but there was no 

 accompanying voltage drop. There seemed to be a 

 general black deposit. On the end of the iron was a 

 metallic globule. The Al electrode was above, and 

 cooled. 



Al-Fe. A rotating mirror revealed a broad band of distinct 

 sparks. The direct appearance was as though a number 

 of arcs were continually jumping across the gap ; in 

 fact it resembled a medium between the ordinary metal 

 arc and the momentary rain of sparks from Cu-Cu in H. 

 The sparks were bluish in colour, accompanied by heat- 

 glow from the metal. The iron appeared little affected 

 but the aluminium boiled considerably, and on making- 

 contact at start the metals tended to fuse together. A 

 kind of crater formed in the aluminium. There was 

 quite a heavy black deposit on the apparatus. 



