396 Cady and Arnold — Electric Arc. 



sometimes on the surfaces of the iron globules. After cool- 

 ing, the platinum rod which had served as cathode looked 

 perfectly bright at the tip, but the platinum anode showed a 

 discoloration, apparently the same as that which de la Rive* 

 took for a sign of oxidation. 



§17. Magnetite-Copper . — The two electrodes belonging to 

 a magnetite arc lamp were used. When the magnetite was 

 positive, the critical point was observed just as with the iron 

 arc. The large size of the electrodes made observations at 

 small currents very irregular.- 



§18, Copper. — After some difficulty, the critical point in 

 free air was found. For comparison with iron, the observa- 

 tions have been plotted in fig. 5. They were obtained with a 

 475 volt supply, length of arc 4 mm , diameter of electrodes 

 6 mm . In fig. 5 only the "parallelogram" mnop correspond- 

 ing to ABCD for iron is shown, so that^?m is a part of the 

 characteristic curve on the first, no on the second stage. The 

 cause of the parallelograms for copper and iron is explained 

 in §30. The change in appearance of the copper arc between 

 the first and the second stage in air is in a general way similar 

 to that for iron, though less pronounced. 



The arc takes place between molten globules of copper 

 oxide, which shrink considerably on cooling. Air cavities are 

 sometimes found inside these globules, probably of the same 

 nature as those in the case of iron. 



§19. Aluminium and Zinc. — The only observations were 

 in free air. The formation of oxides made the arcs so 

 unsteady that no accurate observations could be made. Over 

 a wide range of current no evidence of anything other than 

 the second stage could be found. 



§20. Mercury, Cadmium and Tin. — Arcs were formed in ' 

 air between these metals as anodes and carbon as cathode. 

 The discharge was very unsteady, and no certain indication of 

 the first stage was found. This is not surprising, owing to the 

 low boiling points of the metals. 



§21. Bismuth. — A piece of bismuth was placed as anode 

 in a brass receptacle, with a carbon cathode. Much oxide 

 formed, but it became reduced in the neighborhood of the arc. 

 The arc burned normally on the second stage, but as the cur- 

 rent was decreased, the bright spot on the anode disappeared 

 and the first stage seemed to have set in. Unfortunately the 

 arc could not be kept burning steadily on this stage, and no 

 readings could be taken owing to the irregularity of the dis- 

 charge. 



§22. Lead. — In free air about as much evidence of the 

 first stage could be found as with bismuth. 

 *Pogg. Ann., lxxvi, 270, 1849. 



