72(5 Professor Sir J. J. Thomson [April U, 



AVEEKLY EVENING MEETING, 



Friday, April 14, 1916. 



Colonel E. H. Hills, C.M.G., R.E. D.Sc. F.R.S., Secretary 

 and Vice-President, in the Chair. 



Professor Sir J. J. Thomson, O.M. LL.D. D.Sc.Pres.E.S. M.R.L 



The Genesis and Absorption of X-Rays. 



A YEAR ago I brought before your notice the subject of the produc- 

 tion and properties of very soft Rontgen rays. The rays which 

 emerge from an ordinary X-ray tube are produced by cathode ray& 

 which have fallen through a potential difference of many thousand 

 volts, while the potential difference for the rays I referred to last year, 

 and to which I wish to recur this evening, varies from 20 to 5,000 

 volts. I will begin by considering the method of producing the rays 

 and also of the voltage from which they derive their energy. The 

 source of the rays was a tungsten spiral of the kind used in Coobridge 

 tubes, and I am indebted to Mr. Coobridge for his kindness in pre- 

 senting me with a supply of these spirals. These spirals are heated to 

 incandescence by an electric current, and give out streams of cathode 

 rays, which fall through the voltage between the hot spiral and an 

 anode and acquire energy proportional to the voltage. To get rays- 

 of a definite type it is necessary to have means of producing a constant 

 potential difference. One method I have used for doing this is a 

 battery of a large number of small storage cells ; another method, 

 which, except for very small voltages, is more convenient and more 

 flexible, is to use a high-tension dynamo designed so that the potential 

 when the speed is constant is not subject to appreciable variation. The 

 one I have used was supplied by Messrs. Evershed and Vignolles, it 

 gives steady voltages from 150 to 5,000 volts, and has proved very 

 satisfactory. 



I now pass on to consider the method of studying the character 

 of tlie Rontgen radiation produced at different voltages. Two methods- 

 Jiave been employed : (1) the absorption of the radiation by thin 

 layers of different substances, such as tin, silver, gold, aluminium,, 

 celluloid, has been measured ; (2) in this method the quantity deter- 

 mined was the velocity of the cathode particles which are emitted by 

 a plate of metal when exposed in a vacuum to the radiation. 



AVhen Rontgen rays fall on a plate of metal it emits cathode 

 ])artic]es and the speed of these particles, while independent of the 

 kind of metal, increases with the hardness of the rays; indeed there is. 

 considerable evidence to show that the energy of the particles is, 



