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Prof. J. A. Fleming. Electric Discharge [Jan. 9, 



molecules carry negative charges of electricity, and when they impinge 

 upon a metal plate placed in the vacuum they can discharge them- 

 selves if this plate is positively electrified, either by being in metallic 

 connexion with the positive electrode of the lamp or with a separate 

 positively charged body. When the plate is simply insulated the 

 stream of negatively charged carbon molecules brings down this 

 insulated plate to the potential of the base of the negative leg, or to 

 the potential of that part of the carbon conductor from which it is 

 receiving projected molecules. These carbon molecules projected 

 from an incandescent conductor can carry negative charges, but either 

 cannot be positively charged, or else lose a positive charge almost 

 instantly when projected off from the conductor. 



In the case of the electric arc we must suppose that the negative 

 carbon is projecting off a torrent of negatively electrified carbon 

 molecules, and these, impinging against the positive carbon, wear out 

 a crater in it by a sand-blast-like action. 



The higher temperature of the positive carbon in a continuous 

 current arc is thus explained as due to the impact of the carbon mole- 

 cules projected from the negative carbon. 



If the electric arc is diverted against a third insulated lateral 

 carbon, the carbon blast from the negative carbon wears out a crater 

 in it and brings it down to the same potential as itself. The actions 

 going on in an electric arc may be considered to be somewhat as 

 follows : — When the carbons are first put together, the resistance at 

 the point of contact renders the extremities incandescent. When 

 thus incandescent and separated, the electrification of each carbon is 

 sufficient to begin the projection of molecules from both positive and 

 negative carbons, probably most largely from the latter. The impact 

 of the molecular stream from the negative pole raises the tempera- 

 ture of the positive carbon, and this again by radiation raises the 

 temperature of the negative carbon end. The electromotive force is 

 thus able to keep up a projection of negatively charged carbon mole- 

 cules from the end of the negative carbon, which molecules are 

 loosened from the mass by heat, and then move away by electric 

 repulsion from the surface in virtue of the electric charge which they 

 retain. It would seem as if a hot carbon molecule cannot retain a 

 positive charge, and hence the potential difference between a third 

 insulated carbon and the positive carbon of the arc is nearly the 

 same as the potential difference of the positive and negative carbons 

 of the arc. The rise of potential along the arc takes place very sud- 

 denly just in the neighbourhood of the crater of the positive carbon. 



It has often been suggested that the electric arc contains a 

 counter-electromotive force. It is questionable whether such experi- 

 ments as those of Edlund (' Phil. Mag.,' vol. 36, 1868, p. 352) are 

 entirely conclusive oj. this point. 



