474 DR. CHARLES G. BARKLA ON POLARISED RONTGEN RADIATION. 



that a change of position of the bulb by any such angle did not appreciably affect the 

 result, so that by using an entirely different portion of the primary radiation no 

 sensible difference in the results was detected. But, perhaps, the most conclusive 

 proof of all was given when lead, copper, iron and such heavy metals were used as 

 radiators. The shape and size of these were exactly those of the paper and aluminium 

 radiators, yet when they were used no variation in intensity was detected. 



This result must be connected with the fact that from these metals the radiation 

 differs considerably from the primary which produces it, while from paper and 

 aluminium the radiation bears a much closer resemblance to the primary. 



In addition to these experiments many changes were made in the apparatus 

 without affecting the results described. Three or four different X-ray tubes were 

 used to produce the radiation, though all were of the same type,* the sizes of 

 apertures were varied considerably, the whole apparatus was taken down and 

 reconstructed, and the distances between screens were varied. 



Having given the experimental evidence that the variation in intensity was due 

 simply to the fact that from certain light radiators placed in such a primary beam the 

 intensity of secondary radiation depended on the position of a plane passing through 

 the axis of the beam and fixed relative to the bulb, the maxima and minima being in 

 directions at right angles, that is, that the primary beam is partially polarised, we 

 will consider some of the results a little more in detail. 



Many experiments were made with paper or cardboard as the radiator, and not one 

 failed to give evidence of polarisation in the manner described. 



Table I. gives an example of readings obtained. 



As will be seen from these results, there was a small variation in the ratio of the 

 deflexions of the electroscopes when all controllable conditions were kept the same, 

 but this was small and irregular compared with the variation depending on the 

 position of the bulb. 



When the kathode stream was vertical, the deflexions representing the vertical 

 and horizontal intensities were 16 '8 5 and 13 '6; when the kathode stream was 

 horizontal they became 18 '6 and 12 '15 respectively. 



The numbers taken absolutely have no special significance, as they depended on 

 the construction of the electroscopes, but the changes due to change of position of 

 the bulb show the change in intensity of secondary radiation due to rotation of the 

 primary beam. 



When aluminium was used as the secondary radiator similar results were obtained, 

 the polarisation being well marked. Some of the readings are given in Table II. 



The secondary radiation proceeding from air was also studied, but for various reasons 



the results were more inconstant. The ionization produced in each of the secondary 



electroscopes in a given time was very much smaller than that produced by the 



radiation from a solid, consequently the possible error due to irregularities in the 



* Cox's " Record Tubes " were used in these experiments. 



