CHAPTER VI 



THE CONDUCTION OF ELECTRICITY THROUGH 



GASES 



" It is difficult to think of a single branch of the physical sciences 

 in which these advances are not of fundamental importance. . . . 

 The physicist sees the relations between electricity and matter laid 

 bare in a manner hardly hoped for hitherto. . . . But it is the 

 philosopher that these researches will affect most profoundly. As 

 much by the aid of a perfect mastery over the properties of materials 

 as by the sheer intellectual power of abstract reasoning, some 

 of the fundamental problems of the constitution of matter are here 

 presented as on the verge of solution." — Times^ 22nd January 1904. 



Unlike the liquid solutions and other electrolytes 

 studied in the last chapter, gases, in normal 

 conditions, are almost perfect insulators of elec- 

 tricity. Telegraph wires are insulated by the air 

 which surrounds them, and, if leakage occurs to 

 any measurable extent, it can always be traced to 

 the solid supports to which the wires are attached. 

 Nevertheless, by delicate instruments, a slight 

 leakage of electricity through air can be detected. 

 This air leakage is usually extremely small, but it 

 can be increased greatly in many ways. The 

 passage of Rontgen rays, the incidence of ultra- 

 violet light on a metal plate, the neighbourhood 

 of flames, incandescent metals, or of radio-active 

 bodies such as radium, are among the agencies 

 whereby the condition of the surrounding air is 

 modified so that it can rapidly conduct away the 

 electric charge. 



In general, the currents through gases are too 



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