Negative Ions by Glowing Metallic Oxides. 89 



If this view is correct, then the limiting current must in- 

 crease in proportion to the number of ions emitted by the 

 oxide. But the number of corpuscles emitted by the oxide 

 is directly proportional to the measured saturation current ; 

 so that the limiting current should increase in direct proportion 

 to the saturation current. An examination of the experimental 

 results from this point of view showed the correctness of this 

 conclusion. 



IV. Applications of Glowing Metallic Oxide Cathodes 

 in the Construction of Discharge Tubes*. 



1. When the glowing oxide cathodes are at a very high 

 temperature, the limiting current-density reaches a value of 

 several amperes per unit of area [cm. 2 ] . These high limiting 

 current-densities enable us, by the use of glowing oxide 

 cathodes having areas of several square centimetres, to send 

 much stronger currents through the discharge-tubes than has 

 hitherto been possible. This is due to the fact that with 

 ordinary tubes the cathode drop even with feeble currents 

 already far exceeds the normal value of 300 volts. The con- 

 ditions are essentially different in the case of the strong 

 currents which may be sent through mercury vapour. 



The fall of potential along the positive column is, as has 

 been shown by special measurements, under low pressures 

 and within the range of 0*1 to 15 amperes, entirely in- 

 dependent of the current and lies between 1 and 2 volts per cm. 

 More exact measurements on this point are not yet available, 

 as the heating of the walls of the tube by the strong currents 

 liberates a large amount of gas from them, whereby the 

 pressure and with it the potential gradient along the positive 

 column is greatly increased. 



The small drop along the positive column, combined with 

 the absence of a drop at the cathode, renders it possible to 

 work fairly long tubes by means of voltages such as are 

 furnished by electric light circuits [110 to 220 volts]. The 

 regular positive layers which are formed with strong currents 

 glow with extraordinary brightness, and promise to supply a 

 valuable aid in the examination of gas spectra in spectrum 

 analysis. The ultra-violet rays might be allowed to escape 

 through quartz windows cemented into the tube. 



2. If in a discharge-tube the anode be brought close up to 

 the glowing oxide cathode, the positive column disappears, 

 and there remains only the anode drop of some 20 volts ; 

 i. e.j a potential-difference of little over 20 volts is sufficient 



* The tubes here described are obtainable from E. Gundelach, 

 Gehlberg-, in Thuringia. 



