Manchester Memoirs, Vol. Hi. (1908), No. ^. 



IX. "A Method of Counting the Number of a Particles 

 from Radio-active Matter." 



By Professor E. RUTHERFORD, F.R.S., 



AND 



H. Geiger, Ph.D. 



( Received and read Febfuary nth, igoS.) 



The total number of a particles ex[)elled per second 

 from one gram, of radium has been estimated (Rutherford 

 Phil. Mag., Aug. 1905) by measuring experimentally the 

 total positive charge carried by the a rays from a thin film of 

 radium, on the assumption that each a particle has the 

 same charge as an ion produced in gases. If the a particle 

 is an atom of helium, it is necessary to assume that each 

 a particle carries twice the ordinary ionic charge. The 

 need of a method of directly counting the number of a 

 particles shot out from radio-active matter has long been 

 felt in order to determine with the minimum of assump- 

 tion the charge carried by the a particle and also the 

 magnitude of other radio-active quantities. 



It can be calculated that an a particle expelled from 

 radium produces about 80,000 ions in a gas before its 

 ionizing power is lost. With very sensitive apparatus, it 

 should be just possible to detect the ionization produced 

 by a single a particle by electrical methods. The effect, 

 however, would be small and difficult to measure with 

 accuracy. In order to overcome this difficulty, we have 

 employed a method which automatically increases the 

 ionization produced by an a particle several thousand 

 times and so makes the electrical effect easily observable 



March 14th, igo8. 



