Cady and Arnold — Electric Arc. 385 



as partially reflected from a piece of plate glass. Back of 

 the glass, at the same distance as the arc, was a vertical paper 

 scale, brightly illuminated. The arc was then seen superposed 

 upon the scale. 



Beside German silver rheostats, a large carbonate of soda 

 resistance was used to secure a finer regulation of current. 



§3. The Iron Arc in free Air. — In order to make the 

 transition from the second to the first stage it is of prime 

 importance that the arc shall have settled into a normal state. 

 "When the arc has just been struck, with fresh electrodes, oxida- 

 tion immediately commences, resulting in the formation of a 

 mass of molten magnetic oxide on each terminal. Owing to the 

 comparatively rapid consumption of the cathode, the molten 

 oxide here presents a flat or even concave surface ; at the anode 

 on the other hand the oxide forms a rounded globule of con- 

 tinually increasing size, due to the transportation of matter 

 from the cathode. It was found best to touch the terminals 

 together from time to time, thereby transferring a part of the 

 positive globule to the cathode and causing the discharge to 

 take place between convex surfaces. "When cold, the oxide 

 tips are easily detached from the iron electrodes, leaving gener- 

 ally a blunt point of iron at the anode and a slight concavity 

 at the cathode. 



Thus the normal iron arc in free air is in reality one between 

 terminals of magnetic oxide of iron. A test for iron nitride in 

 the globules gave negative results. Air-holes of considerable 

 extent are often found in the cooled positive globule. The 

 origin of these is not certain, but an examination of the globules 

 indicates that air finds an entrance at points around the edge 

 between the globule and the metal, making its way to the 

 neighborhood of the arc, where owing to the highly liquid state 

 of the globule it emerges, leaving a cavity behind. The same 

 is true of copper (see §18) and of iron when the arc has been 

 burned in nitrogen. When the vapors from the iron arc con- 

 dense, brown oxide forms in a copious powder. Most of these 

 details are doubtless familiar to all who have worked with the 

 iron arc, but it seems worth while to refer to them in view of 

 the important relation borne by the oxidation to the critical 

 point. 



§4. In free air, with iron terminals from three to six milli- 

 meters in diameter, the arc is generally on the first stage as 

 long as the current is below one ampere. The arc proper has 

 then a bluish-white color at the negative end, which gradually 

 shades into dark blue as the anode is approached. At the 

 anode is a layer that seems almost completely dark. A thick 

 yellowish-green mantle surrounds the arc, apparently due to 

 the oxidation of the iron in the arc itself. This mantle seems 



