454 Messrs. Pollock, Wellisch, and Ranclaud on 



4. Effect on the Relighting of Changes in the Relative 

 Positions of the Carhons. 



The form of the relation between the minimum potential- 

 difference for relighting and the time interval between 

 the break and the make of the circuit is influenced by 

 many conditions. With the carbons in a vertical plane 

 they may be situated, before the circuit is broken, either in 

 the normal way, with the crater above, or in the reverse 

 position with the crater below the negative electrode ; in 

 addition, the connexions to the battery may be reversed 

 during the interval between the break and the make of the 

 circuit, or left unaltered, so that there are four cases to be 

 considered in connexion with the relative positions of the 

 carbons and the direction of the potential- difference on the 

 remake of the circuit. 



In figure 4 (PL IX.) are shown the relations between the 

 minimum potential-difference for relighting and the time 

 interval of interruption of the circuit for the four cases 

 mentioned, the current having been 10 amperes when the 

 connexions were broken and the arc-length 1/3 millimetres. 

 The diagrams of the carbons drawn beside each curve 

 indicate by their shape the relative positions of the electrodes 

 before the circuit was opened, while the signs of the potentials, 

 on the reclosing of the circuit, are shown by the usual symbols. 

 All the observations were taken with the same pair of carbons, 

 so the curves are strictly comparable. 



Case 4 is the one just discussed in section 3. It is seen 

 that for small time intervals between the break and the make 

 of the circuit, it requires greater potential-differences to 

 restart the arc in the cases 1, 2, and 3 than in that of 4, 

 and that cases 2 and 3 approximate to that of 4 for large 

 intervals of time. Considering that the arc develops from a 

 small non-luminous current of negative ions, an idea which 

 we think must form the basis of any explanation of the 

 critical relighting, in searching for a description of the 

 differences between the curves it has to be noticed that in 

 some cases the negative electrode, on the remake of the 

 circuit, is hotter than the positive, in others the reverse ; it is 

 also essential to recognize that in some instances the negative 

 stream of ions is opposed by the convection current of hot 

 gas, while in others it is helped by it, as it has been shown 

 in the previous paper that a change in the relative directions 

 of the stream and current considerably affects the potential- 

 difference necessary for arcing. 



The feature of the relations is the evidence, shown by the 



