68 



THE ELECTRIC ARC 



very luminous when in the arc, and copper is used for the 

 anode because it is not rapidly burnt away and does not 

 need to be renewed as often as a carbon anode. The anode 

 is large and the rapid conduction of heat by the copper 

 keeps it at a comparatively low temperature. This arc is 

 very efficient and the electrodes need little attention be- 

 cause of their slow consumption. 



246 



CURRENT !N AMPERES 



FIG. 17. 



Characteristic curves for this arc are given in Fig. 17. 

 The equation, 



, . 25 + 140 



=33 + 24.5^ + - j- 3 



which is of the same general form as those given by Mrs. 

 Ayrton, agrees very closely with these curves. 



Dyott 1 gives characteristic curves for an arc having a 

 copper anode and a cathode consisting of 60 per cent 

 magnetite (Fe 3 O 4 ), 27 per cent titanium oxide (Ti0 2 ) and 

 13 per cent chrome iron (FeCr 2 O 4 ). His values of the volt- 



1 Elec. World, 51, 854; 1908. 



