On the Ionisation of Various Gases by the 

 a Particles of Radium. 



By W. H. Bragg, M.A., Elder Professor of Mathematics and 

 Physics ill the University of Adelaide. 



[Read April 3, 1906.] 



In a paper "On the Recombination of Ions in Air and 

 other Gases" (Tr.R.S.S.A., vol. xxix., p. 187), Mr. Kleeman 

 and I have described the preliminary steps of an enquiry into 

 the total ionisation produced in different gases by the a 

 particle of radium, and the influence thereon of the physical 

 conditions of the experiment. With the assistance of Mr. J. 

 P. V. Madsen, B.Sc, I have made a number of experiments 

 in continuation of the enquiry. It is necessarily a lengthy 

 one, and in some respects difficult, so that on many of the 

 points involved no definite conclusions are yet within reach. 

 On others, results have been obtained which are, I think, of 

 some interest and importance. In this paper I propose to 

 describe the work which has been done : and, in addition, to 

 make some reference to (a) the magnetic deflection of the a 

 particle, ( b ) its acquirement of a positive charge. 



As described in the paper referred to, the total ionisa- 

 tion of a gas can be measured in terms of the product of the 

 co-ordinates of a certain point on the ionisation curve. The 

 true measure is, of course, the area between the curve and 

 the axes of co-ordinates. But experiment shows that all 

 ionisation curves due to radium in radio-active equilibrium 

 are of the same form, and differ from each other only in the 

 application of some factor to all their ordinates or all their 

 abscissae. Thus the product of the co-ordinates of some stan- 

 dard point is proportional to the area of the whole curve, and 

 may be taken as a relative measure of the total ionisation. 



In all the experiments to which I am about to refer the 

 a particles cross at right angles a shallow ionisation cham- 

 ber whose upper electrode is a brass plate and the lower a 

 brass gauze ; the distance between the electrodes is about 

 3 mm. An electromotive force of 300 volts is usually applied 

 to the gauze, giving an electric force of 1,000 volts per cm. 

 This is usually sufficient to ensure saturation, that is to say, 

 to avoid all errors due to diffusion, general recombination 

 and initial recombination. When it is insufficient, the pro- 

 per correction is found and applied. The ionisation cham- 

 ber is enclosed within a vessel which is satisfactorily air- 

 tight except at higher temperatures, and this again within 

 an electric oven. The gas under observation can thus be sub- 



