the Relighting of the Carbon Arc. 455 



curves for cases 2 and 3, of a critical change in the conditions 

 for relighting occurring when the potential-difference attains 

 the value of 90 volts. After reaching this value the mini- 

 mum potential-difference requisite to start the arc remains 

 for some time practically constant in spite of the fact that 

 for increasing time intervals between the break and the 

 make of the circuit the temperatures of the carbons are 

 diminishing. 



The excess of the potential -differences for relighting 

 required in case 1 over those in case 4, for the same time 

 intervals, may be accounted for, perhaps, by the much lower 

 temperature of the negative electrode on the reclosing of the 

 circuit in the former instance, but we have been unable to 

 find, w T ith the data at hand, an explanation of the critical 

 characteristic of the curves for the other cases. 



5. Critical characteristic as affected hij Arc Lengtli 

 and previous Current. 



With a view to finding the influence of arc-length and 

 previous current on the critical characteristic of the curves 

 just mentioned, further observations have been made in con- 

 nexion with case 3, in which the carbons are in the normal 

 position and the battery connexions remain unaltered 

 during the interval between the break and the make of the 

 circuit. The measurements are given in figures 5, 6, and 7 

 (PI. IX.), the currents previous to the break of the circuit 

 being marked on the curves. The observations were 

 thoroughly interlocked with reference to change of carbons, 

 so the curves are comparable. 



A comparison of the relations given in figures 5, 6, and 7, 

 shows that the characteristic bend in the curves is more pro- 

 nounced the higher the previous current. Greater currents 

 mean larger masses of heated carbon with consequent higher 

 temperatures for equal intervals between the break and the 

 make of the circuit. 



It is also seen that the greater the arc length, the higher 

 the potential-difference at which the characteristic bend 

 occurs. 



G. Photographs of the Relighting. 



By arranging an additional lever, in connexion with the 

 pendulum apparatus previously described ; to open the shutter 

 of a photographic camera at a short interval after the 

 remaking of the connexions of the arc circuit, photographs 

 showing stages in the development of the arc have been 



