﻿362 Prof. E. Rutherford on the Mass and Velocity of 



Mass of the a particles from Actinium. 



In order to obtain a homogeneous source of a rays, the 

 active deposit of actinium was used. The active deposit was 

 concentrated on a small copper plate by making it the 

 negative electrode in a small vessel containing the eman- 

 ating actinium compound. This active deposit consists of 

 two products, actinium A and B, the former of which is 

 ray less. The activity imparted to a plate, ten minutes after 

 removal from the emanation, decays exponentially with a 

 period of 36 minutes. The rays emitted are all of one kind 

 and have a range in air, found by Dr. Hahn in this laboratory, 

 of 5*5 cms. 



The preparation of actinium * employed was not very 

 active, and the activity imparted to the copper plate was too 

 weak to produce appreciable photographic action at the 

 distances required. With the experience gained in the 

 previous experiments with weak radioactive sources, it was 

 recognized that at least tw r enty active wires, placed successively 

 in position, would have been required to produce a measur- 

 able photographic effect in the magnetic deflexion apparatus. 

 Three or four times this number would have been necessary 

 in the electrostatic experiment. In order to avoid the 

 necessity of such a procedure, the apparatus was constructed 

 so that the copper plate could be kept active in the position 

 required for any length of time. The actinium compound, 

 wrapped in thin paper, was placed round the sides of a 

 small brass vessel, which was attached to the base of the 

 magnetic or electric deflexion apparatus. A small insulated 

 copper plate, with its plane slightly inclined to the vertical, 

 was placed below a narrow r slit covered with mica in the 

 base of the apparatus and was kept negatively charged. 

 The activity on the plate reached a maximum after three 

 hours and then remained constant. The radiation passing- 

 through the slit into the magnetic deflexion apparatus was 

 mainly due to the a rays from actinium B. The photo- 

 graphic effect of the radiations from the emanation close to 

 the active plate was too weak to be observed. The magnetic 

 deflexion of the pencil of the a. rays was determined under 

 identically the same conditions as in the experiments using 

 radium C as a source of rays. The photographic plate was 

 exposed for ten hours in a constant magnetic field which was 

 reversed at intervals. Two fine w^ell-defined lines were 

 obtained on the plate. The amount of the magnetic deflexion 



* I am indebted to Mr. H. Lieber of New York for his kindness in 

 lendirg me the sample of actinium used in this experiment. 



