648 Dr. H. Greiger and Mr. J. M. Nuttall on Ranges of 



The apparatus, which is slightly modified, is shown again in 

 fig. 1. The main part consists of two large bell jars A A and 

 BB separated by a brass plate CC. 

 A laroe number o£ holes — about 

 250 — each 3' 7 mm. in diameter 

 and 8 mm. high, forming a circular 

 " grid " of 10 cm. diameter, were 

 drilled through the brass plate CC. 

 An exactly similar but much thinner 

 brass plate DD could be screwed 

 on to the lower side o£ plate CC. 

 Between these two plates a thin and 

 uniform sheet of mica equivalent in 

 stopping power to 1*1 cm. of air 

 was placed, and fixed down air- c 

 tight. The lower bell-jar BB con- 

 tained the active film FF at a 

 distance 22'5 cm. from the mica. 

 On account of the metal grid only 

 the a particles emitted at angles 

 less than 20° to the normal could 

 pass through the mica into the 

 upper bell-jar, which contained the 

 electroscope. The latter was cylindrical in shape and was 

 8 cm. high and 10*5 cm. in diameter. It could be 

 charged from outside through the wire R, which could be 

 rotated by means of the ground-glass joint T. In the 

 experiments the upper bell-jar was first completely exhausted 

 and then filled with hydrogen up to a pressure of about 

 6 cm. of mercury. Hydrogen was used in preference to 

 air, since, according to Taylor * and others, the rise of the 

 ionization curve is more pronounced in the former gas than 

 in the latter. When filled with hydrogen to 6 cm. pressure 

 the depth of the ionization vessel corresponded to 1*6 mm. 

 of air at atmospheric pressure, since the u particles had to 

 travel through about 8 cm. in the electroscope before they 

 were stopped by the upper plate. The ionization at different 

 parts of the range of the a particles could be measured by 

 varying the pressure in the lower bell- jar. In this way the 

 ionization curves for the a particles from the different 

 products were obtained. 



The active materials were again deposited on ground-glass 

 plates in exceedingly thin films. In order to prevent the 

 escape of emanation into the bell-jar BB, which would have 

 completely vitiated the experiments, the active film was 

 covered with a thin and uniform sheet of mica attached to a 



* T. S. Taylor, Phil. Mag-, xviii. p. 604 (1909) ; xxi. p. 571 (1911). 



