450 Messrs. Pollock, Wellisch, and Ranclaud on 



and the object of our experiments has been to find the re- 

 lation between this latter factor and the time interval, for 

 carbon-carbon arcs in air at natural pressure, under various 

 conditions. 



The maximum time of interruption of the circuit, under 

 given conditions, within which the arc will reform on re- 

 making the connexions, is astonishingly well-defined, and 

 could in our observations be determined to *002 second. If 

 the interval between the break and the make of the circuit 

 exceeds what may be called the critical time for the given 

 circumstances^ after reclosing the circuit a small non- 

 luminous current passes between the carbons; the heating 

 effects associated with this current are not sufficient to> 

 maintain the electrodes at their high temperatures, and the 

 current soon dies away as the temperatures diminish. From 

 considerations advanced in a previous paper* with reference 

 to the establishment of the cathode fall of potential which is 

 such a characteristic feature of the developed arc, one is led 

 to think that, on reclosing the circuit, this smaller current 

 always precedes the larger one of the fully formed discharge; 

 the problem of critical relighting is then essentially that of 

 the change from a non-luminous to a luminous current under 

 the circumstances of the experiments. 



In the relighting of the arc both carbons are at a high 

 temperature, and the conditions are complicated by the pre- 

 sence, at the moment of reclosing the circuit, of ions at the 

 anode surface as well as near that of the cathode. Simpler 

 conditions are associated with the change of current regime 

 when only the negative carbon is incandescent; this case, 

 involving, previous to the formation of the arc, the flow of 

 negative electricity from a hot to a cool carbon, has been 

 investigated by two of us f, and an explanation reached which 

 seems to account for the phenomena observed. 



In the present experiments, it will be seen that the flow of 

 negative electricity at the moment of reclosing the circuit is 

 not always from a hot to a cooler carbon ; the conditions of 

 the change from the non-luminous to the luminous discharge 

 are, therefore, in some instances, more complicated than 

 those in the case previously considered, and the explanation 

 of the development of the arc suggested in the paper just 

 mentioned is not sufficient to account for all the features 

 observed in this investigation. Further data are required 

 before a complete description can be given. 



* Pollock & Ranclaud, Phil. Mag-. March 1909. 

 t Pollock & Ranclaud, he. cit. 



