372 Prof. Pollock and Mr. Ranclaud on the 



negative ions near the hot carbon, and requires a knowledge 

 of the relation between the mobility of the ions and distance 

 from the hot surface, which depends on the temperature 

 gradient existing between the carbons. 



6. Change from the Non-luminous to the Arc Discharge. 



In the carbon arc there is a very characteristic cathode 

 fall of potential which indicates an accumulation of positive 

 ions near the cathode surface. In view of the fact that, in 

 the experiments under discussion, the arc forms long before 

 the current is saturated, while there is yet a potential rise at 

 the cathode, and therefore no possibility of ionization by 

 collision near the hot surface, one must look to the anode 

 for the origin of the change from the non-luminous to the 

 arc discharge. As a confirmation of this view we have 

 noticed that the change to the luminous regime of current 

 flow is always heralded by the appearance of a small white- 

 hot spot somewhere on the face of the positive carbon. After 

 the advent of the spot, the development to the full arc takes 

 place too rapidly for its phages to be followed by the eye,, 

 although if the circuit is immediately opened, the formation 

 of the luminous discharge may be prevented. 



If I is the current of negative ions, E the potential gradient 

 at the anode surface, and A, 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 A, ; it is 

 here suggested that for the arc to form, the potential- dif- 

 ference between the carbons must reach a 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. These ions, in travelling to 

 the cathode, first annul the hot carbon rise of potential, and 

 then by accumulating near the cathode surface create a 

 cathode fall of potential. With a cathode fall the positive 

 ions bombard the cathode surface with considerable energy, 

 thereby raising its temperature. As a result the electrons are 

 projected in greatly augmented numbers, and with enhanced 

 velocity, their speed being still further increased during the 

 free flight in the now reversed field. When the velocity of 

 the electrons, at the end of their average range, reaches the 

 value of 2' 6 x 10 8 centimetres per second, it may be considered, 

 as suggested by one of us *, that the arc discharge is fully 

 established. 



In our experiments, the currents when arcing commences- 



* Pollock, supra, p. 361. 



