842 Mr. G. A. Hemsalech and the Comte de Gramont 



on 



continued over a long range has shown us that the speed of 

 the electrode in our experiments is in the neighbourhood 

 o£ one metre per second. This result allows us to evaluate 

 the order of magnitude of the duration of the various phases 

 in the arc flash. Thus the principal events in the course 

 of development of the 80 volts water arc flash between 

 magnesium poles would be as follows : — ■ 



Moment of separation of electrodes 00000 second. 



End of first phase 00001 „ 



Abrupt drop in intensity of X 4481 0-0015 „ 



End of arc flash 00048 „ 



In a similar manner the principal epochs during the 

 period of existence of an 80 volts water arc flash between 

 cadmium electrodes are : — . 



Moment of separation of electrodes 0*00000 second . 



End of first phase 0*00009 „ 



First drop in intensity of spark lines X 5338 



and X 5379 00013 



End of emission of spark lines and of 



enhanced line X 4416 0'0024 



End of emission of arc line A 4413, marking 



extinction of flash 00050 ,, 



During the first phase of an arc flash the spark lines 

 attain a development which, as regards character and relative 

 intensity, approaches that observed in powerful condenser 

 sparks. It is therefore of interest to compare the duration 

 of this phase with that of an electric spark. From photo- 

 graphs of condenser discharges through air at atmospheric 

 pressure taken upon a moving film by one of us, in con- 

 junction with Sir Arthur Schuster *, we have calculated 

 that that period of the spark which comprises the initial 

 discharge and the rapid oscillations through the metal 

 vapour lasts approximately 0*000016 second. It is during 

 this short interval of time that spark lines are most strongly 

 emitted. Now we have just seen that the time taken by the 

 upward-moving electrode to pass through the liquid film of 

 a water arc is about 0*0001 second, so that some of the 

 phenomena which happened during the early stages of 

 the progress of the electrode tip through the liquid film 

 must have been as short-lived as the oscillating phase of a 

 condenser spark. 



* Schuster and Hemsalech, Phil. Trans. Roy. Soc. vol. exciii. p. 189' 

 (1899). 



