*2 IONISATION OE GASES [CH. vn. 



(ind Geitel by which they examined in great detail 

 a number of incandescent metals in the form of a 

 wire or strip heated by an electric current in different 

 gases. In J. J. Thomson's variety of experiment 

 this current was produced in the secondary of a 

 transformer, so as to be conveniently insulated. The 

 details are long and complicated, and must be referred 

 to in J. J. Thomson's book on The Conduction of 

 Electricity through Gases \ but briefly it may be 

 said that when the gas in the vessel is air, the metal 

 plate A receives a positive charge when the wire is 

 heated to a dull red glow ; as it becomes hotter the 

 charge increases until the wire is at a yellow heat ; 

 if it is made hotter than that, the charge diminishes, 

 and, at the highest temperature, is very small. In 

 hydrogen the plate is found to receive negative elec- 

 tricity when the wire is hot enough, though at a lower 

 temperature it receives positive. To get rid of 

 occluded gases, J. J. Thomson often kept it red hot for 

 a week or more. McClelland, Branly, H. A. Wilson, 

 and others have investigated this phenomenon, which 

 has also been studied in a different form by Preece 

 and Fleming following up a curious observation 

 made by Edison in connexion with incandescent 

 lamps. Kecent measurements by 0. W. Eichardson 

 have done much to reduce it to a definite physical 

 specification. The evidence that an incandescent 

 platinum wire disintegrates, or at any rate gives off 

 material, is the fact discovered by Aitken, that a 

 cloud becomes visible in a luminous beam, if the 

 moist gas surrounding the wire be suddenly cooled. 

 This indeed can occur at a much lower temperature 

 than incandescence. Mr. Owen (Phil. Mag., Sept., 

 1903), found that when a platinum wire was in air 

 there was always a cloud when the temperature of 



