152 PHYSICS OF THE ELECTRON 



The extremely varied aspects which this discharge takes, the pro- 

 duction of striations, an explanation of which was first given by 

 J. J. Thomson, the influence of a magnetic field on the conditions of 

 the discharge, the phenomena that are produced when the electrodes 

 are only of the order of a micron apart, where the molecules do not 

 appear to take part in the production of the spark, are many of the 

 essential points which to-day attract attention. 



(46) The Electric Arc. By the side of the ordinary disruptive 

 discharge, by brush or spark, the electric arc, with an entirely differ- 

 ent aspect, brings in the new phenomenon of the emission of cathode 

 corpuscles by the surface of incandescent bodies. This incandescence 

 of the electrode, of the cathode especially, is, in fact, characteristic of 

 the arc discharge; the cathode is raised to a sufficiently high tem- 

 perature by the impact of the positive ions which flow toward it, 

 so that the corpuscles present in the electrode, and which give it its 

 conductivity, experience a true evaporation and carry the greater 

 part of the current. In fact, a filament of incandescent carbon is able 

 to emit, at a much lower temperature than that of the voltaic arc, 

 cathode corpuscles representing a current density of two amperes 

 per square centimeter. 



(47) Evaporation of the Cathode. This phenomenon, known under the 

 name of the Edison effect, is very general and has been connected in 

 a quantitative manner by Richardson on the fundamental hypothesis 

 of the kinetic theory with the presence of freely moving cathode 

 particles in the interior of conductors. 



At ordinary temperatures this emission of corpuscles is diminished 

 to such an extent that electrostatics is possible and a metal can 

 keep a permanent charge. Every corpuscle present in the metal is 

 immersed in a medium of high specific inductive capacity, and a 

 finite amount of work is necessary to make them pass from this 

 medium to a region where the specific inductive capacity is equal 

 to unity. Only the corpuscles having a sufficient velocity would be 

 able to supply this work on leaving the conductor, and their num- 

 ber, absolutely negligible at ordinary temperatures, increases with 

 extreme rapidity with the rise in temperature. Richardson has 

 shown that the variation obtained by experiment agrees very well 

 with that predicted by theory. 



(48) Metals. The spontaneous dissociation of atoms which the 

 kinetic theory implies, the separation of electrified centres free to 

 move in the interior of the metal, is a consequence of the high specific 

 inductive capacity of the medium, of the ease of electrostatic polar- 

 ization of metals, owing to the ease with which the metallic atoms 

 lose corpuscles in order to remain positively charged. The potential 

 energy of an electrified particle in such a medium is much smaller than 

 anywhere else, and conformably with the laws of the distribution of 



