172 T. S. Taylor— Retardation of " Alpha Bays." 



A, and the brass plate, B. The gauze A was carefully insu- 

 lated from the plate B by means of an ebonite ring G. To 

 prevent any possible leakage from A to B a fine copper wire 

 was run in a groove around the inner edge of this ring, then 

 through the ring and around its outer edge, and, after passing 

 through the large ring of ebonite R, to the earth. A second 

 gauze C was placed 3 mm below A and, being earthed, formed 

 with it a second ionization chamber which prevented stray ions 

 from entering the main chamber. 



The plate B was connected to one pair of quadrants of a 

 Dolezalek electrometer as shown in figure 2, the second pair 

 of quadrants being earthed. K is a key by means of which 

 the one pair of quadrants and plate B could be earthed or 

 insulated at pleasure. K rested on a brass plate M, which in 

 turn was insulated from the protecting case I by being placed 

 upon a block of sealing wax .N . 



The polonium, which was used as the source of rays, being 

 put upon the plate V, could be moved towards or from the 

 ionization chamber by means of the screw W, of l mm pitch, 

 working in the nut X. Its distance from the chamber could 

 be determined by the scale L and the graduated circular disc 

 r. The metal sheets were placed upon the brass ring c, c 

 and could be moved to different positions by means of the 

 screw H, of l mm pitch, working in the nut T. The distance 

 of the metal sheet from the polonium could be read from the 

 scale L by means of the pointer P, which extended below the 

 case F from the moveable framework cinL. 



The polonium kindly prepared for me by Prof. Boltwood 

 was deposited upon the end of a copper plug, which could be 

 fitted into a block of brass and adjusted to a definite distance 

 from the top of the block. The opening in the block above 

 the polonium was 4 mm in diameter and 6 mm high. This limited 

 the cone of rays given off by the polonium so that all the rays 

 would fall within the ionization chamber. 



The ebonite was carefully polished to prevent leakage over 

 its surface. The entire framework of the apparatus together 

 with the tin case F and the protecting case I for the connec- 

 tion of the plate B to the electrometer was grounded. Care 

 was taken to avoid all outs.ide electrostatic effects upon the 

 electrometer as well. 



Method of Experiments. 



The block containing the polonium was put upon the disc 

 V and a metal sheet on the ring c, c, which was then adjusted by 

 means of the screw H until the sheet just touched the top of 

 the block, thus being 6 mm from the polonium. With a con- 

 stant potential of —40 volts on the gauze A and the metal 

 sheet at a distance of 6 mm from the polonium, the Bragg ion- 

 ization curve was plotted. 



