n;oi>rn:i> r.v HOT PI.ATINTM IN DIFKKKKXT CASKS. 



Besides oxygen, the present paper contains an accotmt of measurements of the 

 ionisation of Ix-tli signs from liot platinum in air, nitrogen, helium, and hydrogen. 

 There are also measurements of the ionisation from a platinum surface in air when a 

 calculable quantity of hydrogen is diffusing out from the interior of the platinum. 

 The last-named experiments shed a considerable amount of light on the mechanism of 

 the processes by which the ions are produced. 



It is necessary to say a word about the use of the term ionisation in this paper. 

 By "the positive (or negative) ionisation" in a quantitative sense is meant the 

 number of positive (or negative) ions lilx-ratcd by 1 sq. centim. of the platinum 

 surface per second under the conditions specified. The ionisations are, therefore, 

 proportional to the respective saturation currents calculated per square centimetre of 

 surface. 



For convenience of reference the paper has been subdivided as follows : 



I. 1. Introduction. 



II. 2. Experimental arrangements. 

 III. The ionisation in oxygen : 



3. Current and electromotive force. 



4. Hysteretic relations between current and E.M.F. 



5. Current and pressure. 



6. Current and temperature. 



7. Uncontrollable variationa 



8. Comparison of different wires. 



9. Special properties of new wires. 



10. Theory of the steady positive leak in oxygen. 



IV. 11. The ionisation in nitrogen. 



V. 12. The ionisation in air. 



VI. 13. The ionisation in helium. 



VII. 14. The ionisation in hydrogen. 



VIII. 15. Experiments with a platinum tube. 



IX. 16. Theoretical considerations. 



X. 17. Summary of principal results. 



II. 2. E-NPKRIMENTAL ARRAN<;KMKNT8. 



K \rept where the contrary is distinctly stated, the arrangement of apparatus used 

 was similar to that in the author's previous papers.* The platinum wires were 

 supplied by Messrs. JOHNSON MATTHKY AND Co., and were of the purest material 

 obtainable. They were O'Ol centim. in diameter and were in the form of a loop, the 

 wire being about 7 centims. long. The ends of the loop were welded on to stouter 

 platinum leads (A, fig. 1) which were sealed into one end of a glass tube about 



* Cf. ' Phil. Trans.,' A, vol. 202, p. 243. 



i: '-' 



