DR. CHARLES O. BARKLA ON POLARISED RONTOEN RADIATION. 



471 



Electroscope A, differed from these in having its lower face of paper and aluminium 

 instead of one of the side faces, so that it could be used to measure the intensity of a 

 vertical beam of X-rays. 



The rod of each electroscope was connected to one terminal of a battery of Leclanche' 

 cells, whose other terminal was earthed so that the insulated wire and gold-leaf of 

 each electroscope could be charged by means of the 

 contact maker, which momentarily connected the 

 rod and wire, afterwards leaving the wire and gold- 

 leaf charged and insulated. 



There was a small leak in each electroscope, due 

 to the normal ionization of the air within the case 

 containing the gold-leaf. The support leak through 

 the sulphur, which must have been excessively 

 small, was from the rod kept at constant potential 

 to the wire and gold-leaf. In the steady normal 

 state, when the potential was high enough to 

 produce a saturation current, the leak measured 

 the normal ionization of the air within the case. 

 The cases and screens were all earth -connected 

 during the experiments. 



The radiator first experimented upon consisted of 

 sheets of paper approximately square in shape, but 

 slightly modified so that a pencil of the primary 

 radiation passed through an opening almost at the 

 centre of the square, then through the aperture C 

 in the lead screen S 6 into electroscope B. The 

 radiator was held in position by a fine supj>ort of 



Fig. 2. 



wood, cardboard, or aluminium. Paper was first chosen because experiments had led 

 to the conclusion that the secondary radiation from such a light substance was very 

 similar to that from gases, and, according to the theory given, should vary in intensity 

 in any direction at right angles to that of propagation of the primary beam with a 

 change in the position of the plane of polarisation of that beam. 



The position of the radiator was such that the whole of one side was exposed to the 

 primary radiation, the angle of incidence of which varied in different experiment, and 

 electroscopes A t and A., were exposed to secondary radiation proceeding from the 

 same face, also in oblique directions. 



When a beam of Rontgen radiation was sent through the two apertures, GI and Cf, 

 the rates of deflexion of the three electroscopes were considerably increased. By 

 placing lead screens at the different apertures and again determining the rates of 

 deflexion when the discharge was sent through the X-ray tube, it was seen that the 

 gold-leaves in electroscopes A, and A, were deflected by secondary radiation proceeding 



