Negative Ions by Glowing Metallic Oxides. 



87 



exploring electrode and the cathode is, no doubt, due to the 

 fall along the positive column], but that it rapidly increases as 

 soon as the characteristic current is exceeded. This current 

 I have named the limiting current. 



Since the limiting current increases with rise of tempera- 

 ture I have investigated the relation connecting the two 

 quantities somewhat more closely. 



Connexion betiveen Limiting Current and Temperature. — 

 By slowly increasing the current at various temperatures, that 

 value of it was determined beyond which the cathode drop 

 just commenced to increase — i. e., the limiting value of the 

 current. The relations so obtained between the limiting 

 current and temperature are nearly identical, qualitatively as 

 well as quantitatively, for the oxides of calcium, barium, and 

 strontium ; hence, in this case also, I give, in Table IV=, the 

 results obtained with CaO only. 



• 



Table IV 



. 











Relation connecting Limiting Current with Temperature 

 (CaO cathode). ^> = about O'Ol mm. Hg. 





94^ 



10121075 



11331192 



1?5? 



1313 



1367 



1485° 



Limiting Current, 10— 3 amp. . 



o-i 



0-56 1-68 



5-6 11-8 



26* 



35-4 f 



56-1* 



700* 



Limiting Current - density, 1 

 in 10-3 amp. cm. -2 j 



024 



1-2 3-57 



i 



12 25 



55 



75 



120 



1500 



In the third row of the table are given the limiting current- 

 densities, i. e., the limiting currents per unit of area [icm.~ 2 ] 

 of the glowing oxide. As will be seen, the current-density 

 reaches a very large value at high temperatures. 



The curves drawn in fig. 4 have been plotted from Table IV. 

 With increasing pressure the limiting current decreases, but 

 otherwise the relation connecting limiting current with 

 temperature remains unaltered f. 



Meaning of Limiting Current. — As already mentioned above, 

 according to modern views the cathode dark space represents 

 a region poor in negative ions. If into it we introduce cor- 

 puscles generated by any suitable method, the cathode drop 

 will decrease. If now we increase the current through the 

 discharge-tube, we do away with more negative ions, the 



* With large currents, the walls of the tube became strongly heated, 

 so that the pressure could no longer be kept constant, but increased 

 almost to 1 mm. 



f A. Wehnelt, Ann. d. Physik, xiv. p. 451 (1904). 



