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IX. On the Number and the Absorption by Matter of the 

 P Particles emitted by Radium. By Walter Makower, 

 Demonstrator and Assistant Lecturer in Physics in the 

 University of Manchester *. 



WHEN radium is in radioactive equilibrium with its 

 pro lucts of disintegration, it is known to emit 

 x, /3, and 7 rays. The a rays are emitted by the radium 

 itself, the emanation, radium A, and radium C ; whereas 

 the /3 rays are emitted by radium C and by radium B onlyf. 

 Now it is known that the /3 rays consist of negatively charged 

 particles projected with liigh velocity, whereas the 7 rays 

 carry no charge with them ; so that a mass of radium so 

 situated in an insulated enclosure that only the /3 and 7 rays 

 can leave it, should lose a negative charge, and therefore 

 become charged with positive electricity. That this is the 

 case has been shown by M. & M me Curie J, Strutt §, and 

 others. It is a matter of importance to ascertain the 

 magnitude of the negative charge carried away from a 

 known quantity of radium in radioactive equilibrium, and 

 hence to deduce the number of /3 particles emitted. 



The first attempt was made by Wien ||, from whose data 

 it may be deduced that, if each /3 particle carries a charge 

 of 3'4 x 10~ 10 electrostatic unit, one gram of radium emits 

 1*13 x 10 10 ft particles per second. The method of expe- 

 riment consisted in suspending in a very perfect vacuum 

 a known quantity of radium in an insulated platinum vessel 

 connected to an electrometer, and measuring the charge 

 acquired by the electrometer in a known time. The value 

 obtained is, however, certainly too low, since no account 

 was taken of the absorption of the rays by the walls of the 

 platinum vessel. A subsequent series of experiments was 

 carried out by Rutherford ^[, in which this source of error 

 was eliminated. A lead rod, 4 centimetres long and 4 milli- 

 metres in diameter, was made active by exposure to radium 

 emanation for a sufficient time to obtain the maximum 

 activity. After testing the quantity of active deposit thus 

 collected on the lead rod by comparison with a standard 

 quantity of radium, the rod was covered with just sufficient 

 aluminium-foil ('053 millim. thick) to absorb the a rays, 

 and transferred to a vessel in which the charge acquired by 



* Communicated bv the Author. 



t H. W. Schmidt, Phi/s. Zeit. vi. p. 897 (1905). 



j M. & M™ Curie, Comptes Rendus, cxxx. p. 647 (1900). 



§ Strutt, Phil. Mag. Nov. J 903. 



|| Wien, Phys. Zeit. iv. (1903). 



% Rutherford, Phil. ftlng. Aug, 1905. 



