of Secondary Cathode Rays produced by X-Rays. 39 



connected to A and B respectively, and the wire C was 

 normally connected to earth. A third ionization-chamber, 

 D, received some of" the rays from Rj and was always 

 connected to earth. A wire running from the inside of D 

 and insulated from it connected with the gold leaf of a 

 second electroscope, E 2 , of the ordinary type. This wire 

 and gold leaf could be charged to any convenient potential ; 

 and the rate of fall of the gold leaf when the rays were 

 turned on served as a measure of the intensity of the rays 

 from R L . Each electroscope was fitted with a microscope and 

 scale for observing the motion of the gold leaf. A lead 

 screen, F, could be slid across the window in front of B by 

 means of a screw, giving a very fine adjustment of the 

 amount of the radiation entering this chamber. 



The chambers A and B consisted of brass cylinders about 

 10 cm. in diameter and 2'2 cm. long, covered with lead on 

 the outside. The ends of B were covered with aluminium 

 foil. The end of the wire C which entered A terminated 

 in a wire ring of a diameter slightly less than the inside 

 diameter of A, and lying in a plane perpendicular to the 

 axis of A. The opening in the screen S opposite A was so 

 adjusted that no part of the beam from R : could fall on the 

 ring or on the walls of A. 



Brass plate-holders, E and I, covered the ends of A. 

 These plate-holders were made exactly alike to be inter- 

 changeable. Each one consisted of two square brass plates, 

 held together with screws, and having a circular hole bored 

 through their centre of about the same diameter as the 

 inside of A. These plate-holders could be slid on and off 

 the ends of A in somewhat the same manner as a camera 

 plate-holder, and stops were so placed that they would always 

 come to the same position, with the centre of the hole on the 

 axis of A. 



In order to produce the emergence cathode rays it was 

 necessary to pass the rays from Ri through a plate of some 

 metal in which cathode rays are produced in sufficient 

 numbers to be easily measured. As these metals are rela- 

 tively opaque to X-rays it was necessary to use very thin 

 sheets. Grold and silver were fixed upon for this purpose. 

 The gold leaf was 0*8 x 10 ~ 5 cm. thick and the silver 

 leaf 1*8 x 10" 5 cm. Circular disks of aluminium about 

 0*012 cm. thick were made with a diameter slightly larger 

 than the holes in the plate-holders, and on one side of each 

 was laid a thin foil of one of the above metals, stuck on 

 with a thin coat of vaseline. When thicker sheets of metal 

 were wanted more foils could be added by first holding the 



