180 Dr. C. Sheard on the Positive 



after the wire had been re-inserted in the apparatus filled 

 with dried air. The values o£ the initial currents obtained 

 after heating at various temperatures, the wire being put to 

 earth, for 10 minutes in each case, are shown in Curve III. of 

 fig. 1, and the slow-decay values in Curve IY. Curve III. 

 is in form a repetition of Curve I. and shows maxima at about 

 the same temperatures. It is to be borne in mind that these 

 effects were obtained by heating in the gas-flame following 

 the low current values given in the tabulation at the be- 

 ginning of this paragraph ; hence the absorption of hydrogen 

 from the flame has in some manner produced a large increase 

 in the ionization subsequently obtainable. Under further 

 treatment the current-temperature relations become com- 

 parable with those given in the above tabulation. The wire 

 was again heated in the burner for 10 minutes. Curve V. 

 of fig. 1 and Table I. give data showing the effects produced 

 by the second heating. The thermionic currents were in- 

 l creased but little in comparison with the effects produced by 

 the first heating in the Bunsen flame as shown in Curve III. y 

 fig. 1 ; the maximum effect at about 756° C. is, however, 

 clearly indicated. 



The proof is conclusive that heating in hydrogen revivifies 

 I tne wire and produces an increased positive emission. The 

 role which such a gas plays, however, has been variously 

 explained. The recent work of Professor 0. W. Richardson * 

 has quite definitely settled this question in one of its aspects, 

 for he finds that there is no emission of gas ions and that the 

 values of e/m remain the same before and after heating in a 

 gas. The increased emission produced by hydrogen is pro- 

 bably in part due to the removal of material, incapable of 

 producing ions, built up at the surface of the wire. This 

 removal may be occasioned by the mechanical effects of the 

 gas on diffusion into or evolution from the wire, or may be 

 due to its reducing action. The ionization effects following 

 the cleaning of an " old " wire in nitric acid t indicate the 

 existence of such a non-ionizing substance. 0. W. Richardson 

 has shown that "merely straining the metal will cause a 

 revival of the positive emission," and that the renewal 

 of emission under various circumstances is probably mainly 

 a mechanical effect. 



An important function played by hydrogen, however, 

 may be attributed to its affinity for electrons. Franck J has 



* Proc. Royal Society, A. vol. lxxxix. pp. 507-524. 



t Sheard and Woodbury, Physical Review, vol. ii. pp. 288-298 (1913). 



% Verh. d. D. Phys. Gesell. xii. pp. 291 & 613 (1910). 



