196 Prof. E. Rutherford on the Charge 



leak without a magnetic field shows that the electrons are 

 projected with such slight velocity that they cannot travel 

 against the electric field. In a later experiment, where 

 the plates were closer together, it was found that the electro- 

 scope gained a positive charge in a. strong maguetic field, 

 proving that the a rays carried a positive charge. 



The experimental method used by the writer was not 

 so much for the purpose of detecting the charge carried by 

 the a rays, as to measure this charge and so deduce the 

 number of a. particles expelled from a known quantity of 

 radium. The use of radio-tellurium as a source of a rays is 

 very advantageous, as the ft rays are altogether absent. It has 

 been shown, however, that in the case of radium the disturb- 

 ance due to the j3 rays can be avoided by using radium at its 

 minimum activity. 



An account will now be given of the details of the experi- 

 ments. 0'484 milligram of radium bromide was taken, 

 dissolved in water, and the solution evaporated uniformly on 

 a polished aluminium plate, about 20 sq. cms. in area. This 

 amount of radium bromide was not directly weighed, but 

 determined by comparison of the 7 ray effect in an electroscope 

 with that produced by 23'7 milligrams of radium bromide. 

 The latter had previously been used in experiments to deter- 

 mine the heating effect of radium, and gave out heat at a 

 slightly greater rate than 100 gram-calories per gram per 

 hour. The radium bromide was thus probably pure. 



Assuming that the radium was uniformly distributed on 

 the plate, the weight of radium bromide per sq. cm. of surface 

 was 2*4 x 10 -5 grams. This film of radium was so thin that 

 only a very small fraction of the a particles shot out from 

 the surface was absorbed in the radium itself. This is seen 

 to be the case, when it is remembered that half of the a 

 particles emitted by radium are stopped by about '0004 cm. 

 of aluminium, that is, by a weight of aluminium per unit 

 area of about '001 gram. Assuming that the absorption is 

 proportional to density, only a few per cent, of the rays can 

 have been absorbed by the radium itself. 



The saturation current due to this deposit of radium at its 

 minimum activity was measured between plates 3*5 cms. 

 apart by means of a high-resistance galvanometer. The 

 current observed was 8*4 x 10~ 8 amperes. 



A second set of experiments were made with a glass instead 

 of an aluminium plate. The deposit of radium was covered 

 with a layer of aluminium-foil "00034 cm. thick. The 

 saturation current, observed as in the case of the aluminium 

 plate, corresponded to 3*3 X 10~ 8 amperes. From this, it was 

 •concluded that the number of a particles issuing through the 



