456 On the Relighting of the Carbon Arc. 



obtained. The growth of the arc takes place in such a short 

 time that a definite adjustment o£ the lever was found to be 

 impossible, and the photographs given are only a few of many 

 that have been taken, the remainder showing either no arc, 

 or the arc fully developed. 



Figures 1 to 7 in Plate X. show stages in the growth of 

 the arc when the circuit is opened and reclosecl without 

 alteration of the battery connexions, while figures 8 to 11 

 refer to cases where, on the reclosing of the circuit, the still 

 existing crater is negative to the previous cathode. In both 

 instances the glow is seen to develop from the electrode which 

 is positive on the remake of the connexions, a fact which 

 seems to support the view taken in this and the previous 

 paper as to the mode of growth of the arc from the non- 

 luminous discharge. Figure 12 is an example of many of 

 the photographs, showing that, in the case of the reversal of 

 the connexions, the new crater commences on cool rather 

 than on hot carbon. 



7. Summary. 



In connexion with the relighting of the carbon arc, without 

 movement of the electrodes, when the circuit is opened and 

 reclosed, the relation between the potential - difference, 

 established between the carbons at the moment of the 

 remaking of the connexions, and the maximum time of inter- 

 ruption of the circuit, within which the arc will reform, has 

 been investigated for cases differing as to the relative positions 

 of the carbons before the opening of the circuit, and as to the 

 direction of the potential-difference after the reclose of the 

 connexions. The problem is that of the change from a non- 

 luminous to a luminous discharge in air at normal pressure, 

 of which an explanation, in the case where negative electricity 

 flows from a hot to a cool carbon, has been given in a previous 

 paper. In the present experiments both carbons are at a 

 high temperature, and the conditions of the change are com- 

 plicated by the presence, at the moment of reclosing the 

 circuit, of ions at the anode surface as well as near that of 

 the cathode. In some of the cases examined the relations 

 show a critical characteristic, but sufficient data are not 

 available to enable an explanation of this result to be given. 



We are indebted to Mr. H. L. Watkins, B.A., and Mr. L. A. 

 Cotton, B.A., B.Sc, for help in connexion with the earlier 

 part of the investigation. 



The Physical Laboratory, 

 The University of Sydney, 



September 2nd, 1908. 



