Thermionic Currents with Potentials. 553 



To test the second possibility — viz., that lack of satu- 

 ration was due to ionization in a dense vapour near the hot 

 surface — a wire was mounted in a glass cylinder containing 

 a cylindrical receiving electrode and a quantity of coconut- 

 charcoal in a space below the open end of the cylindrical 

 electrode. Liquid air could be applied to the charcoal and 

 also to a trap in the pump-connexion just outside the glass 

 cylinder. Observations were taken with liquid air off and 

 on the charcoal, off and on the trap, and off and on both 

 together. There was no detectable change in the value 



Ai 



of — . The pressure on the McLeod gauge was 1x10"° 



before applying liquid air, and no change occurred upon 

 applying the liquid air. No change in the gauge-reading 

 was expected, however, because the vapour, if it existed, 

 would condense on the colder portions of the tube before it 

 reached the gauge. The result of this experiment indicated 

 that there could be no considerable amount of vapour 

 around the hot surface. The fact that there was no appre- 

 ciable cooling of the hot wire when liquid air was applied 

 was a further indication that there was very little gas 

 present. 



Since the first two hypotheses concerning the location of 

 the phenomena causing lack of saturation were apparently 

 untenable, there remained only the hypothesis that the effect 

 was due to phenomena at the hot surface. To get at the 

 nature of these phenomena, a detailed study of the pecu- 

 liarities connected with initial emissions was undertaken. In 

 all twenty-three different wires were studied under various 

 conditions of purity, surface-cleanness, pressure, &c. It 

 w r as known that water vapour actively modifies thermionic 

 currents, so water vapour was in all cases excluded from the 

 apparatus. In parts of the experiments the wires were 

 sprayed with salts, and in some cases they were heavily 

 coated with CaO or BaO. 



The results of the series of investigations may be sum- 

 marized as follows : — In addition to the well-known time 

 decay and variation with potentials, there were two features 

 that characterized all of the emissions studied. The first of 



Ai 

 these was an initial growth in the value of — . This growth 



was rapid at first, and more gradual later. A stationary 

 value often occurred followed by a period of slow decay. 

 The initial growth was repeated for each succeeding higher 

 temperature. The rate of subsequent decay was more rapid 

 for high than for low temperatures. 



