the Relighting of the Carbon Arc. 453 



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 connexions, 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 fig. 3 may be considered as giving, 

 approximately at least, 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 particlular case. From this point 

 of view one would expect the curves in fig. 3 to be like those 

 in fig. 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 considered to develop. 

 In the fully formed arc the cathode fall of potential indicates 

 an accumulation of positive ions near the cathode surface. 

 These positive ions, in the growth of the discharge from the 

 non-luminous current, must come in the first instance from 

 the anode. If I is the current of negative ions, E the 

 potential gradient at the anode surface, and X the length of 

 the last free run of the ions at the end of which they collide 

 with the anode, the energy reaching the anode surface per 

 second is IE\ ; following the suggestion contained in the 

 paper referred to, it is considered that for the arc to form, in 

 the circumstances here contemplated, the potential-difference 

 between the carbons, at the moment of reclosing the circuit, 

 must have the value necessary to make the magnitude of 

 IEA, sufficient to raise a portion of the anode surface to such 

 a temperature that positive ions are somewhat freely 

 emitted. 



For this particular experiment the suggestion seems 

 sufficient to account for the phenomena observed, but it will 

 be seen that it is not of itself adequate to completely describe 

 the features of the relation in other cases, even where, on 

 reclosing the circuit, the hot is negative to the cooler carbon. 

 * Pollock & Ranclaud, he. cit. 



