540 Prof. A. S. Mackenzie on the Deflexion of 



pole-pieces of the electromagnet to be placed at any part of 

 its length and as close together as possible. The wedge- 

 shaped part of the tube was to allow for the deflexion of the 

 rays ; but this expansion of that part of the box and the 

 size of the cylinder D were made large, in order to prevent 

 as far as possible secondary radiations from the walls of the 

 box from affecting the photographic plate. The disk was 

 bored almost through to receive the slit-tube, and the bottom 

 was pierced by a slit 3 mm. long parallel to the slits in the 

 slit-tube when the latter was in its proper position. This 

 slit was covered on the outside by a piece of mica about 

 •0006 cm. thick through which the a rays passed into the 

 vacuous box. The mica was put on with sealing-wax, and 

 was as thin as would safely withstand a difference of pressure 

 of an atmosphere on its two sides. The slits were J mm. 

 wide, and 5*75 cm. apart. The circular opening in the end 

 of the cylinder D was covered with the glass zinc-sulphide 

 screen, powdered side inwards, put on with sealing-wax. A 

 light-tight cover E fitting over the end of D served to make 

 this end a sort of camera. The distance from the zinc 

 sulphide to the nearest slit is 15*4 cm. 



The radium was contained in, and filled, a shallow depres- 

 sion in a disk of brass, R, of the same diameter as A. This 

 cell was attached to A when required, and the joint covered 

 with soft wax to prevent the escape of emanation into the 

 room. As the radium filled the depression in the cell, it was 

 practically in contact with the mica when it was attached to 

 the box, and thus the distance through which the a rays had 

 to go in air at atmospheric pressure was very small. 



A photographic impression of the line of scintillations 

 when undetected by the magnet could be obtained in two or 

 three hours; but when deflected the exposure had to be 

 increased several times, due in part to the absence in this 

 case of the /3 and <y rays and in part to the dispersion of the 

 beam of a ravs. 



In order to see whether the u particles always retain their 

 charge when they travel, and whether the scintillations are in 

 part due to uncharged particles, the magnetic field was applied 

 over lesser and greater extents of the path of the rays and to 

 different parts of the path. For this purpose it was necessary 

 that the path of the rays from their source to the far end of 

 the slit-tube should be always shielded completely from the 

 magnetic force. This was satisfactorily attained by winding 

 round this part of the box many turns of a thin ribbon of 

 varnished soft iron. 



Measurements were made with four different arrangements 



