178 Prof. A. Stanley Mackenzie on Secondary Radiation 



the plate of the electroscope were brought to the required 

 high potential by connecting with one pole of a battery of 

 small storage-cells, the other pole of which was earthed, in 

 the usual way. The axis of the cylinder made an angle of! 

 23° with the horizontal. 



The radiation entering the ionization-cell could be absorbed 

 by screens S placed against the aluminium end of the cell. 

 The position and inclination of the cylinder were so arranged 

 that the radium lay in the plane of the aluminium-leaf, as 

 shown by the dotted line cd : this is to reduce to a minimum 

 the secondary radiation set up in the screen S by the <y rays 

 from the radium, and was sufficient to make any leak due to 

 this cause so small as to be negligible. 



When a plate was to be bombarded it was put either in 

 position R or in position T. Position R was such as to make 

 equal angles with the central axes of the lead block and the 

 ionization-cylinder, in which position the reflected radiation 

 entering the cell was found to be a maximum. It is inclined 

 toward the lead block about 10° from the vertical. The 

 distance from the radium to the plate measured in the line 

 of the axis of the block was about 13 cm., and from the 

 aluminium-leaf to the plate, measured along the axis of the 

 cylinder, about 6 cms. Position T was horizontal, and 

 the lower surface of the plate 8 mm. below the corner of 

 the block b. The distances from radium and aluminium, 

 measured as before, were 18 and 12 cm, respectively. The 

 plates were 30 x 40 cm. in area, and of various thicknesses. 

 It was found that the same effect was produced by a single 

 plate of given thickness as by a pile of plates of the same 

 aggregate thickness. 



The secondary radiation from the front side of a plate has 

 been carefully investigated by Eve"*, McClelland f, Allen J, 

 and others, and the chief properties of these rays are now 

 beyond doubt. The interpretation of the results of experi- 

 ments of this kind is very difficult on account of the complex 

 character of the radiations involved, and what seems to be a 

 penetrating radiation may be easily confused with one which 

 is only of a tertiary nature, and so on. It was hoped that in 

 the apparatus as here set up the various radiations could be 

 kept fairly well distinct. 



The stream of radiation which enters the cylinder, especially 

 when either the reflecting plate Hor the transmitting plate T 

 is used, is made up of many components, consisting of 

 (1) 7 rays direct from radium, and the secondary rays they 



* Phil. Mag. Dec. 1904. % Phys. Rev. Aug. 1906. 



t Phil. Mag. Feb. 1905. 



