RELIGHTING OF THE CARBON ARG. 921 
In figure 3 is shown the relation between the minimum 
potential difference for relighting and the interval between 
the break and the make of the circuit, under the condition 
of the reversal of the potentials of the carbons, for a 
previous current of 10 amperes. Three curves are drawn, 
from observations with the same pair of carbons, for arc 
lengths of 1°3, 1°9 and 3°1 millimetres respectively, the 
upper electrode being the positive before the break of the 
circuit. 
In this instance, on reclosing the circuit, the hot is 
negative to the cooler carbon; the conditions under which 
the arc is formed are therefore nearly allied to those in the 
simple case previously investigated, see section 1. The 
longer the interval between the break and make of the 
connections, the lower are the temperatures of the carbons 
at the moment of reclosing the circuit; taking the fall of 
temperature of the carbons after the arc is extinguished 
as nearly proportional to the time, the curves in figure 3 
may be considered as giving, approximately at ieast, the 
form of the relation between the critical potential for 
relighting and the temperature of the hot negative, the 
temperature of the other electrode being of less importance 
in this particular case. From this point of view one would 
expect the curves in figure 3 to be like those in figure 7 
of the previous paper* which give the exact form of such a 
relation under somewhat the same conditions as those under 
consideration. A comparison shows that the two sets of 
curves are similar in shape. 
Under the conditions of the experiment, the first effect 
of reclosing the circuit is no doubt a small current of 
negative ions flowing from the hot to the cooler carbon, 
and from this non-luminous current the arc may be con- 
sidered to develop. In the fully formed arc the cathode 
1 This Journal p. 208. 
