548 W. B. Barss — Measurements of Radio-activity. 



intensities would be constant. Here the ions are distributed 

 throughout the volume of the gas, and general recombination, 

 which depends upon the ionization density, i.e., upon the 

 number of ions per cubic centimeter in the gas, would increase 

 as the ionization itself is increased, unless a saturating field is 

 applied. 



In a great number of important investigations in the subject 

 of radio-activity, it has been assumed that the quantity of 

 radio-active material present was proportional to the ionization 

 currents produced by the a-rays. In these experiments, elec- 

 trical fields have been applied which would have been ample 

 to cause saturation if the ionization had been produced by 

 /3- or X-rays, but which are now known to be quite inadequate 

 to produce saturation when a-rays are employed. Results 

 which have been obtained in this way are of fundamental 

 importance in the theory of radio-active transformation. The}^ 

 include the determination of relative quantities of radio-active 

 substances by the " Emanation Method " and the method of 

 thin films, as well as nearly all the measurements of rates of 

 decay of such substances. It is safe to say that in no case in 

 which such measurements have been made with an electroscope, 

 in air at atmospheric pressure, has a saturating potential been 

 applied, or even very closely approached. The fact that a 

 fairly consistent body of measurements and constants has been 

 built up by many investigators, notwithstanding this apparent 

 flaw in their experimental arrangements, shows that the con- 

 siderations advanced above must have a considerable degree of 

 validity. The object of the present experiments is to test this 

 point specifically in the important case when the a-rays are 

 produced by an emanation mixed with the ionized gas. In 

 this case the sources of the rays are scattered through the gas 

 and on the walls of the vessel, and the paths of the a-particles 

 and their attending columns of ions extend in all directions ; 

 so that the geometrical complication is as great as it can 

 well be. 



We might reasonably expect the ratio of currents to be con- 

 stant in this case, at least for small source intensities. If the 

 number of a-particles is small, there will be only a few columns 

 of ions existing together during the time required for the ions 

 to be carried to their respective electrodes. It is true that a 

 portion of the a-particles will cross each other and that the 

 separated columns of ions will also sometimes cross each other, 

 thus producing some recombination between ions of different 

 columns. But even when this happens, the crossing will usu- 

 ally be at an angle, and the length of each column is so great 

 compared with the diameter of its cross section that even if 

 they do intersect, the amount of this recombination will be neg- 



