﻿138 Prof. E. Rutherford on the Retardation of the 



Retardation of the a Particles in passing through 

 Aluminium. 



The experiments given in a preliminary paper have been 

 repeated with greater accuracy, and by using more active 

 wires and with the photographic plate at a smaller distance 

 from the source, I have been able to determine the velocity of 

 the a particle closer to the end of its range of ionization 

 in air. 



The method adopted was to determine the deflexion o£ a 

 pencil of a rays after passing through successive layers of 

 aluminium-foil. Each layer of foil was found by a separate 

 experiment to be equivalent in stopping power to about 0*50 cm. 

 of air. 



In these experiments, the slit was 2 cms. from the wire and 

 the photographic plate 2 cms. from the slit. This gave a 

 conveniently large deflexion of the pencil of a rays in a mag- 

 netic field of strength about 8000 c.G.s. units. One side of 

 the active wire was bare, and the other covered successively 

 with 2, 4, 6, 8, 10, and 12 layers of aluminium-foil. Although 

 a very strong photographic effect was obtained with the bare 

 wire, the effect through the 12 layers of foil was very weak, 

 but was sufficiently well defined for accurate measurement. 

 The slit was then removed to a distance of 0'5 cm. from the 

 source and the plate to within 1*5 cms. of the slit. The intensity 

 of the photographic impression was consequently increased 

 four times, and the part of the active wire covered with the 

 12 layers gave a well-marked effect on the plate. The amount 

 of deflexion of the pencil of rays was, however, reduced to 

 *375 of that observed at the first distance. The other side of 

 the wire was successively covered with 12 and 14 layers of 

 foil, and the deflexions compared with that due to the original 

 12 layers, used as a standard of comparison. 



A weak though measurable impression was obtained through 

 14 layers. Two layers of Dutch metal, equivalent in stopping- 

 power to "64 of a layer of foil, were then added. A slight 

 darkening of the plate was noted, but was too weak and 

 ill- defined for measurement. 



The results obtained are collected in the following table, 

 where the velocity of the a particles is expressed in terms 

 of V , the velocity of the a particles from the unscreened 

 wire. It will be shown in a later paper that the value of e/m 

 for the a particle is unchanged by its passage through matter, 

 so that the deflexion of the pencil of ra} r s in a magnetic field 

 is inversely proportional to the velocity of the rays. 



