82 Prof. A. Wehnelt on the Discharge of 



galvanometer at various temperatures of the pure or barium- 

 oxide-coated wire when maintained at a potential of + 1000 

 volts. 



These results are exhibited in the curves of fig. 1. The 



Fig. 1 



20 ■* 



80 



60 



.2 bo 



m 20 



Arrip. 











9 

 * 



BclL 































































































































&a6 













; 

















i "*" 











s 



' 















' 



























s, 



w 



Ft 





X«- 



«*. 



U-'l 



—4= 



L--i-^- 



=H3 





/* 1 '/ 





800 



SOQ 



1000 



j400°frteuts 



1100 1200 



Temperature. 



full-line curves show the connexion between i and T for pure 

 platinum wire. From them it will be seen that for tempera- 

 tures below 1450° there are sent out more positive than 

 negative ions*, while at higher temperatures the reverse 

 appears to take place. 



Much more marked than this slight difference between the 

 numbers of the positive and negative ions in the case of pure 

 platinum is that which occurs when the wire is coated with 

 barium oxide (dotted curves of fig. 1) . While the currents 

 corresponding to a positive potential do not appreciably differ 

 from those obtained with a pure platinum wire, the currents 

 with the wire at a negative potential are extraordinarily 

 large even at low temperatures. 



Similar results, both qualitatively and quantitatively, were 

 obtained with the oxides of calcium and barium. 



II. Emission of Negative Ions by Glowing Metallic 

 Oxides at Low Pressures. 



According to the united testimony of all investigators f, a 

 platinum wire heated in vacuo emits principally negative ions. 



* This result is in accordance with those obtained by other workers. 

 Detailed bibliographical references will be found in J. J. Thomson's 

 1 Conduction of Electricity through Gases,' Chapter viii. (1903). 



t J.. Elster and H. Geitel, Wied. Ann. xxxvii. p. 315 (1889). J. J. 

 Thomson, Phil. Mag. xlviii. p. 547 (1899). O. W. Richardson, Proc. 

 Oambr: Phil. Soc. xi. pp. 286-295 (1901) ; Phil. Trans, cci. p. 516 (1903). 

 McClelland, Proc. Oambr. Phil. Soc. xi. p. 296(1991). H. A. Wilson, 

 Proc. Roy. Soc. lxxii. pp. 242-276 (1903), 

 (1903). 



and Phil. Trans, ccii. 



