( 108 ) 



Wehnelt interruptor; the voltage of the battery amounted to 65 Volts 

 and the current to 7 Ampères. A very good photo was obtained in 

 30 hours and it shows very clearly two maxima and two minima, 

 the distance between the centra of the maxima is exactly half the 

 inner circumference of the tube, and it may be deduced from their 

 position that they are caused by the tertiarj^ rays emitted by the 

 conic surface of the carbon bar. 



In this experiment the centre of the anticathode, the axis of the 

 carbon bar and the centre of the carbon plate lay in one horizontal 

 plane, and the axis of the cathode rays was in one vertical plane 

 with the centre of the carbon plate; the axes of the primary and 

 the secondary beams were perpendicular to one another. According 

 to Barkla's supposition we must expect that with this arrangement 

 the maximum of the action of the tertiary rays will be found in 

 the horizontal plane above mentioned. In my experiment this sup- 

 position really proved to be confirmed. In order to know' what part 

 of the photographic film lay in this plane, a small side-tube F was 

 adjusted to the outside of cylinder A, and this tube F was placed 

 in an horizontal position during the experiment. A metal tube with 

 a narrow axial hole fitted in tube F, so that in the dark room, 

 after taking away a small caoutchouc stopper which closed F, I 

 could prick a small hole in the film with a long needle through 

 this metal tube and through small apertures in the walls of A and 

 B. This hole was found exactly in the middle of one of the maxima. 



So this experiment confirms by a photographic method exactly 

 what Barkla had found by means of his electroscopes and it proves 

 that the secondary rays emitted by the carbon are polarised. 



In some of his experiments Barkla pointed out the close agreement 

 in character of primary and secondary Röntgen rays; in my experi- 

 ments also this agreement was proved by the radiogram obtained on 

 the film placed in cover D. Not only did the secondary rays act 

 on the film after having passed through the carbon bar of 6 cm., 

 but also the bright ring was clearly to be seen, which proves that 

 ebonite absorbs all secondary rays which have passed through carbon ^). 

 The ring was not so sharply defined as in the experiments with 

 primary rays; this fact finds a natural explanation in the different 

 size of the sources of the radiation: in the case of the primary rays 

 the source is a very small part of the anticathode, in the case of 

 the secondary rays it is the rather large part of the carbon plate 

 which emits rays through the apertures in G and S^. 



1) The ring was perfectly concentric: the arrangement proved therefore to be 

 exactly symmetrical. 



