Hydrogen Nuclei from Swift a Particles. 901 



represent diametral sections of annular disks described on 

 the equi potential surfaces and having the arrow as a common 

 axis. The lines parallel to the arrow are sections of corre- 

 sponding cylindrical shells. The dimensions below the 

 figure refer to the radii of the annuli. They are given 

 in cms. x 10 -13 . 



The diagram can best be explained by means of a definite 

 example. Suppose a hydrogen nucleus, assumed to be a 

 point, is situated in front of the u particle at P. Then, if 

 the velocity of the a particle is such that the nucleus reaches 

 a potential of 42 million volts during the collision, it will be 

 thrown at an angle of between 20° and 25° of the a. ray. 

 This is indicated in the figure by the annulus on the 

 4*2 million volt equipotential being drawn with its normal 

 inclined at an angle of 22\° to the direction of the arrow. 

 In order that the nucleus may be thrown between 20° and 

 25°, it must lie within a certain area on a plane at right 

 angles to the a-ray. AB in the diagram is such a plane, 

 and the area, determined by calculation from the ex- 

 periments, is that marked out by the cylindrical shell CD 

 on AB. 



The annuli described on the equipotentiais consist of two 

 sets; one derived from curve G (dimensioned on the left of 

 the figure), the other from curve H (dimensioned on the 

 right). The nearest annulus to the axis in each case 

 represents nuclei recoiling between 0° and 15°, the next 

 those between 15° and 20°, the third between 20° and 25°, 

 the fourth between 25° and 30°, and the last between 30° 

 and 35°. 



If interpreted as above, the diagram represents the 

 experimental results correctly whatever the form assumed 

 for the field, provided that it is purely electrostatic. If the 

 recoil of the nucleus is partly due to electromagnetic or 

 other forces, the potentials will not be true electrostatic 

 potentials, but will still describe the maximum potential 

 energy of the collision in a convenient manner. 



If the recoil be due only to electrostatic forces, a rough 

 estimate of the dimensions of the field in the direction of 

 motion of the a particle can be made, and an upper limit set 

 with certainty to distances in this direction. One arrives at 

 this by consideration of the field round a point charge of 

 magnitude equal to that of an a. particle. The 5*5 and 

 3*5 million volt equipotentiais round the point are spheres of 

 radius "52 and -82xl0~ 13 cm. respectively (indicated in the 

 bottom right-hand corner of iig. -I), and it is easy to show 

 that for any distribution of charge other than a point 



