154 Sir E. Rutherford on the Penetrating Power of 



the box, passed into the measuring vessel which was 

 placed close to the opening. The ionization current was 

 measured by means of lead electroscopes of the self-contained 

 type used for gamma rays. Three of these electroscopes, of 

 cubical form, respectively 11 cm., 10 cm., and 12 cm. side, 

 were employed in the course of the experiments. For 

 determining the initial absorption, the lead face of the electro- 

 scope was cut away, and replaced by thin aluminium-foil. 

 For greater thicknesses of absorber, a lead electroscope with 

 sides 3 mm. thick was used; while for still greater thick- 

 nesses, a lead electroscope 8 mm. thick was used in some 

 experiments. In order to avoid disturbances due to stray 

 radiations, the windows of the electroscopes were of thick 

 plate-glass, and still further protected by lead extensions. 

 Such precautions are essential when, as in the present expe- 

 riments, the intensity of the end radiation under measurement 

 was in some cases less than one millionth of its initial value. 



In order to make experiments over such a wide range, the 

 heating current through the tungsten spiral was adjusted to 

 give a convenient rate of leak in the electroscope in each 

 experiment. 



The voltage corresponding to the alternative spark-gap 

 was determined by comparison with the sparking potential 

 between two large brass spheres 20 cm. in diameter. 



The absorbing lead plates, which were of much greater 

 area than the face of the electroscope, were placed close to 

 the electroscope. In such a case, the greater part of the 

 radiation scattered in the absorber in a forward direction 

 enters the electroscope. The average absorption coefficients 

 fi for different thicknesses of the absorber were determined 

 in the usual way. The results for different voltages are given 

 in the following table: — 



Max. 



Voltage. 



Range of 



thickness 



in lead, 



mm. 



Absorption 



Coefficient, 



\i. cm.-l 



Mass 



Abs. Coef. 



ii/ p. 



Max. 



Voltage. 



Range of 



thickness 



in lead, 



mm. 



Absorption 

 Coefficient, 

 /x. cm.-l 



Mass 



Abs. Coef. 



fi/p. 



79,000 



1-8— 2-5 



27 



2-37 



183,000 



0-7— 1-3 



26 



2-28 





2-5—31 



26 



2-28 





1-3— 20 



24 



2-11 



92,000 



1-8—2-5 



25 



2-19 





2-4— 4-0 



20-5 



1-80 





2-5—3-4 



24 



2-11 





4-0— 4-6 



18 



1-58 





3-1—3-7 



24 



211 





4-6— 5-3 



15 



1-32 



105,000 



2'7— 3-3 



23 



2-02 





5-3— 6-4 



13 



1-14 





3-3—3-9 



22 



1-93 





6-4— 7-0 



12 



1-05 





3-9—4-6 



22 



1-93 



196,000 



4-3— 5-5 



13 



114 



118,000 



2-7—3-3 



22 



1-93 





5-5- 6-4 



12 



105 





3-3—3-9 



22 



1-93 





6-4- 7-8 



11 



•96 



144,000 



2-7—3-3 



22 



1-93 





7-8— 9-2 



10 



•88 





3-4—4-6 



22 



1-93 





8-8—10-0 



8-5 



•75 



170,000 



3-1-3-7 

 3-7-4-3 



18 

 17 



1-58 

 1-49 













4-3—5-5 



15 



1-32 











