Discharge of Electricity from Glowing Carbon. 375 



have been estimated, but we have been unable to deduce the 

 magnitude of the potential gradient at the anode surface. 

 The value of this latter factor will include a part due to the 

 projection of electrons from the surface of the anode, which 

 will be greater the higher the anode temperature. 



In the above description it is considered that the develop- 

 ment of the arc from the non-luminous discharge takes place 

 in two stages. The first one commences with the somewhat 

 copious emission of positive ions from the anode surface,, 

 when the energy of its bombardment by negative ions reaches 

 a critical value ; the second one begins when the velocity, at 

 collision with gaseous molecules, of electrons projected from 

 the cathode, commences to increase on account of the presence 

 of positive ions near the cathode surface ; it is completed 

 when this velocity is such as to start a mode of conduction 

 through the vapour column, characteristic of the fully 

 developed arc, in which, perhaps, electrons are handed on 

 from atom to atom through the column, as suggested in the 

 paper just mentioned. 



The view we have taken of the origin of the change from 

 the non-luminous to the arc discharge receives some support, 

 we think, from the observations of the differences in the 

 voltages required for arcing given in Table II. AVhen the 

 hot carbon is vertically below the cool one, the How of mole- 

 cular ions is helped by the convection current of hot gas, 

 whereas in the reverse position the flow is opposed by the 

 current: the value of IE\ may thus, for the same potential- 

 difference, be different in the two cases. In addition, the 

 temperature of the cool carbon when below the hot one is 

 much lower than when above it ; the critical value of IE\ 

 may, therefore, have to be higher for arcing to start in the 

 former case than in the latter. If the change to the luminous 

 discharge i- considered to originate near the cathode surface, 

 the observed differences in the arcing voltages, due to altera- 

 tion in the relative positions of the hot and cool carbons, 

 would be difficult to explain. 



7. Summary. 



The flow of negative electricity from hot carbon, in a 

 circuit containing an air-gap, up to three millimetres in length, 

 between a hot and a cool carbon rod, has been investigated 

 for temperatures of the hot rod from 1100° C. to 1800° C, 

 and for various voltages up to the point at which an arc 

 forms between the carbons, the experiments being made in 

 air at natural pressure. 



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



