282 Mr. Norman Campbell on Delta Rays. 



was sacrificed, and it was clear throughout that the uncer- 

 tainty due to errors of compensation were considerably 

 smaller than errors due to other circumstances over which 

 no control could be attained. 



6. As an indicating instrument a sensitive Wilson tilted 

 electroscope was used. Some experiments were tried with 

 an electrometer with a sensitiveness o£ 5000 divisions for a 

 volt, but no more accurate results could be attained. The 

 extremely short period of the electroscope gives it a great 

 advantage over the electrometer when it is desired to dis- 

 cover, by earthing the electrode, whether there has been any 

 deviation of the potential from that of earth. When the 

 electric field between the electrodes was so great that the 

 current was saturated, the potential which had to be applied 

 to the resistance to attain compensation could be determined 

 with complete certainty to within 0*003 volt. 



7. Experiments were first made to discover whether there 

 was any change of the velocity of the delta rays with a 

 change in the velocity of the exciting alpha rays. For this 

 purpose the form of the relation between potential difference 

 and current was investigated when the hole in B was 

 covered with aluminium leaf of different thicknesses. A 

 was covered with an aluminium plate throughout. The 

 thinnest leaf used was equivalent in surface density to about 

 1 mm. of air ; the thickest through which the rays could 

 be measured was equivalent to about 4'1 cm. Using 

 Rutherford's measurements on the velocity of alpha rays, it 

 appears that in the first case the rays emerging from B and 

 striking A must have had a mean velocity of about 2*0 x 10 9 ; 

 in the latter case of about 1'5 x 10 9 . It must be remembered 

 that the potential required to stop the rays is proportional 

 to the square of their speed, so that, if the velocity of the 

 fastest delta rays had varied in the same ratio as that of the 

 fastest alpha rays, the potential required to stop them would 

 be nearly twice as great with the thinnest foil as with the 

 thickest. Of course the alpha rays were not homogeneous 

 in speed, since some of them passed obliquely through the 

 aluminium leaf, but it is obvious from the values of the total 

 current that the superposition of thicker absorbing layers 

 must have produced a considerable change in the average 

 velocity of the rays. 



8. The results of the measurements are recorded in Table I. 

 The columns marked 1-9 give the observations made when 

 the hole in B was covered with layers of aluminium of which 

 the surface density (grm./cm. 2 ) is given in the first row. 

 The second row gives the current when the difference of 

 potential between A and B was zero ; the third row the value 



