Photograpliic Registration of a. Particles. 621 



easily be obtained, and the magnitudes of the throws 

 remained unchanged for hours at a time. Jn addition, hardly 

 any natural disturbances were observed when the a particles 

 did not enter. In the particular apparatus employed, the a, 

 particles travelled a distance of 3 cm. in the helium. With 

 a pressure of 20 cm. the stopping power of the a. particles 

 in the gas corresponded to only 2 millimetres of air at atmo- 

 spheric pressure. At a pressure of 20 cm. each « particle 

 produced a large electrical effect ; but even when the 

 pressure was reduced to 5 millimetres, the entrance of each 

 a particle could be easily detected by the movement of the 

 fibre of the electrometer. At this latter pressure, the stopping 

 power of the a particle corresponded to only '06 of a milli- 

 metre of air. In these circumstances, about 4U0 volts only 

 were required to give the necessary magnifi cation. These 

 results bring out the enormous increase in the number of 

 ions produced by collision under these conditions. 



It is not possible to count with certainty by eye observa- 

 tions more than about 50 throws of the fibre per minute. 

 The fibre moves so rapidly that it is difficult to recognize 

 when two or more a particles enter the detecting vessel at 

 very short intervals. It is far more certain to use a photo- 

 graphic method for recording the rapid movements of the 

 fibre. For this purpose, we have found the registration 

 apparatus constructed by Professor Edelmann very service- 

 able. The speed of the film could be regulated over a wide 

 range according to the number of a particles to be registered 

 per minute. The types of record obtained for the different 

 rates of entrance of the a particles are illustrated in fig. 2. 



Fi ff . 2. 



-VavuVU\VjUJwU 



The line A shows the part of the record obtained when the 

 film moved at a rate of 13 cm. per minute, and for an 

 average of 50 a particles per minute. The length shown on 

 the photograph corresponds to an interval of about 1 minute. 



