carried by the <x and (3 Rays of Radium. 207 



of radium, and the saturation current produced when the 

 radiation is all absorbed in the gas, we can at once deduce the 

 number of ions produced in air at atmospheric pressure and 

 temperature by the passage of a single a particle through it. 



A weight of '484 milligram of radium bromide spread in 

 the form of a thin film on an aluminium plate emitted 8*7 x 10 G 

 a particles into the gas per second. The saturation current 

 observed between parallel plates at sufficient distance apart 

 to absorb most of the rays was 8*4 X 10~ 8 ampere. Taking 

 the charge on an ion as 1*13 x I0~ 19 coulomb, this current 

 corresponds to a production of 7*5 x 10 11 ions per second in 

 the gas. But this number was produced by 87 x 10 6 a 

 particles. The average number of ions produced by each 

 « particle, expelled from radium itself, is thus 86000. Now 

 Bragg has shown that an a particle passing through air pro- 

 duces nearly the same number of ions per unit length of its 

 path, and the ionization ceases fairly abruptly, The range 

 in air for the a particles from radium at its minimum activity 

 is about 3'0 cms. The number of ions produced per cm. of 

 path in air at normal pressure and temperature is thus 29000. 

 The number per cm. of path at a pressure of one millimetre 

 of mercury would be 38. 



Townsend found that the maximum number of ions pro- 

 duced by an electron per cm. of path at a pressure of one mm. 

 of mercury was 21. At this maximum it was deduced that 

 each collision of the electron with the molecules in its path 

 resulted in the production of ions. Since the a particle pro- 

 duces 38 ions under the same conditions, we may conclude 

 that an a particle is nearly twice as efficient an ionizer as the 

 electron at its maximum efficiency. Such a result indicates 

 that the a. particle has a somewhat larger sphere of action 

 than the electron, and is able to ionize about two molecules 

 for the electron's one. This is not unexpected, since the a 

 particle is of atomic dimension, while that of the electron is. 

 small compared with an atom. 



Energy required to produce an Ion. — A deduction of the 

 average energy required to produce an ion by collision of 

 the a. particle with the gas molecules can be made, if the 

 range of velocity, over which the a particles ionize the 

 gas, is determined. I have shown in my paper* " Some 

 Properties of the a Rays from Radium," that the a rays, 

 emitted by a thin film of radium at its minimum activity, 

 are initially projected with a velocity 'S8 v , where' v is the 

 initial velocity of projection of the a particles from radium C. 

 The a. particles cease to ionize the gas when the velocity falls. 

 * Phil. Mas-. July 1905. 



