Secondary Rant gen Radiation from Carbon. 779 



VII. Description of Apparatus used to Measure the 

 Absorbabilities of the Secondary and Primary Beams. 



The plan of the apparatus is shown in fig. 2. 



Fig. 2. 



The electroscope P x was provided with a lead screen E 

 which had a small circular hole 4 mm. in diameter, allowing 

 a small pencil of primary rays to enter it. The electroscope 

 S provided with a lead tube T, lined with aluminium 0*15 mm. 

 thick, measured the amount of secondary radiation given off 

 by the radiator P. Another electroscope P 2 , not shown in 

 diagram, was placed on the top of the lead box B containing 

 the bulb A, and measured the intensity of the primary rays. 

 The tube T 2 , admitting the rays on to P, was made of lead 

 and was lined with aluminium 0*3 cm. thick. Its dimensions 

 were 6*5 cm. long, 3*3 cm. wide, and 5*5 cm. deep. The 

 tube T\ was 11*5 cm. long, 5 cm. wide, and 4*5 cm. high,- 

 and was distant from the centre of the radiator P 16*5 cm. 

 The window of P 2 was 57 cm. from the anticathode of the 

 bulb. 



VIII. Method of Experimenting \ 

 (a) Measurement of the absorbability of the primary beam. 

 A lead screen with a small circular hole was placed at the 

 side of T 2 nearest the bulb, and was of such a size and 

 position that the central portion only of the primary beam it 

 admitted passed through the orifice of E. The percentage 

 absorption of the primary beam by successive sheets of thin 

 aluminium was then obtained, using the electroscopes P 1 and 

 P 2 . The lead screen, 3 mm. thick at T 2 , was placed there in 

 order to screen E from the secondary radiation from the glass 



