carried by the a. and /3 Rays of Radium. 199 



The negative current was about twice as great as the 

 positive, and over twenty times as great as the current carried 

 by the a. particles alone. 



* A verv similar result was observed when the upper electrode 

 BC was replaced by a thick brass plate, fixed at a distance of 

 about 2 mms. from the lower plate. We must thus cou elude 

 that the escape of electrons from the lower electrode was 

 considerably reduced by placing a layer of aluminium-foil 

 over the radium. The results obtained in the different 

 experiments were fairly concordant. 



The current due to the charge carried by the ex. particles 

 was not small, and the electrometer-needle moved at a con- 

 venient rate when a capacity of '0024 microfarad was added 

 to the system. For example, using the aluminium plate as 

 a source of rays, the mean value of ne was found to be 

 9*8 x 10 _13 ampere. 



Assuming that each a particle carries the same charge as 

 an ion, viz. 3'lxlO -10 electrostatic units, or 1*13 x 10~ 19 

 coulombs, the number of a particles projected per second into 

 the upper plate was 8*7 x 10 6 . This is the number from 

 ••181: milligram of radium bromide ; and remembering that 

 half of the a particles projected from the radium are absorbed 

 in the lower plate, it can be readily deduced that the total 

 number of a. particles expelled per second from 1 gram of 

 radium bromide at its minimum activity is 3* (5 x 10 lu . 



The corresponding number obtained for the *19 milligram 

 of radium bromide on a glass plate was 3*30 x 10 10 , and for 

 the *484 milligram, when the upper plate B was of brass, 

 3' 9 6 x 10 10 . The mean of these three determinations is 

 3*6 X 10 10 , and this may be taken as the probable value. 



Assuming the composition of the compound employed as 

 RaBr 2 , it follows that the total number of <x particles expelled 

 per second from 1 gram of radium at its minimum activity 

 is 6*2 X 10 10 . Now- the a ray activity of radium in radio- 

 active equilibrium is four times this minimum value, and 

 includes three products, viz. the emanation, radium A, and 

 radium C, which emit a rays. We may thus conclude that 

 the total number of a. particles expelled per second from 

 1 gram of radium in radioactive equilibrium is four times the 

 number at its minimum activity, and is equal to 2" 5 X 10 11 . 



This number is in good agreement with the number 2 x 10 11 , 

 previously deduced from direct data, based on the heating 

 effect of radium and the observed volume of the emanation *. 



1 think these experiments show conclusively that the a rays 

 from radium do carry a positive charge. We have seen that 

 * Eutherford {he. cit.). 



