for the Measurement of Recoil Radiations. 



765 



used, one open, and the other covered by an aluminium 

 screen of stopping power equivalent to about 5 cm. of air. 

 The effect is investigated by measuring the difference in 

 ionization produced in an ionization chamber placed above 

 the aluminium window F, when first one of these slots faces 

 F and then the other. Hydrogen from a Kipp generator 

 is passed through the box, by way of the side tubes 1 & J, 

 throughout the experiments. 



Fiff. 2. 



f=l\Q 



The second part of the apparatus (fig. 2), which measures the 

 recoil radiation produced in the box, consists of electroscope, 

 Q, ionization chamber, M, and a compensating ionization 

 chamber, R, used to balance the current through M. P and 

 P' are two vulcanite slabs shaped to fit in the V of the brass 

 box. They support the electroscope and two ionization 

 chambers and insulate each from the others. M and part of 

 1\ are over the window F. The compensating chamber, R, 

 is merely a cylinder of thin brass (183 mgs. per sq. cm. 

 thick) in which slides a piston, S, also of brass, so that the 

 volume of R in which ionization is effective may be adjusted. 

 The ionization chamber at M is a small aluminium box made 

 of sheet aluminium soldered into a 2 cm. cube of ti'24: mgs. 

 per square cm. thickness. The electrode N carrying the 

 electroscope leaf passes from the electroscope through the 

 ionization chamber into P, being supported by sulphur 

 insulation in P and P'. The dimensions of the electrode 

 shown in the figure were arrived at as giving a compromise 

 between complete saturation in the ionization chamber and a 

 low capacity. The electroscope is somewhat similar to one 

 designed by Trepathi, working at the Cavendish Laboratory 

 under Mr. (J. T. R. Wilson. It is, however, a" much rougher 

 and less sensitive instrument. It is made of a brass plate 

 about 2 mm. thick, in which is cut a semicircular hole, U, 

 P3 cm. in diameter. The sides are of thin lead clamped by 

 brass plates of the same shape as the main plate forming the 

 body of the electroscope, bolts passing right through from 

 one side to the other. The leaf is about 3 mm. long and 

 is observed through mica windows, 3 mm. x 1 mm. by a 

 microscope magnifying about #0 diameters. 



