Photographic .Registration of a Particles. 619 



contained air or another gas at a reduced pressure. The 

 cylinder was provided with a central electrode which was 

 connected with an electrometer, and the outer cylinder 

 was charged to a negative potential nearly sufficient to cause 

 the passage of a spark. Under such conditions, the small 

 ionization produced by the a particle is magnified several 

 thousand times by collision, and the entrance of an « particle 

 into the testing vessel is signalized by a ballistic throw of 

 the electrometer-needle. In order to determine the number 

 of a particles emitted per second by one gram of radium, a 

 source of small diameter coated with radium C was placed 

 at some distance from the opening in an exhausted tube, so 

 that a definite fraction of the a particles passed through the 

 mica-covered opening of the detecting vessel. 



Since the electrometer responds only slowly to the rise of 

 potential caused by the entrance of an a particle, it was not 

 possible to count with accuracy more than about 10 a 

 particles per minute. Since our original experiments, several 

 types of string electrometers have been devised. The 

 moving part consists of a thin silvered quartz fibre placed 

 between two charged parallel plates. The fibre follows 

 rapidly any change of potential applied. In 1910 we used 

 a string electrometer, devised by Laby and constructed by 

 the Cambridge Scientific Co., for the purpose of counting 

 a particles. The instrument was found to have sufficient 

 sensibility, and the movement of the fibre produced by the 

 entrance of each a particle into the detecting vessel could be 

 simply recorded on a moving photographic film. 



A number of investigations have been made from time to 

 time in order to find the best conditions for obtaining a clear 

 record of the individual a. particles even when they enter the 

 detecting vessel in very quick succession. The conditions 

 for accurate counting depend on the following : — 



(1) The absence of electric disturbances in the detecting 



vessel when no a. particles enter. 



(2) The equality of throws for each « particle. 



(3) The elimination of the effect of scattering of the a 



particles in the mica window and the gas in the 

 detecting vessel. 



For accurate work, it is essential that the fibre after 

 displacement should return rapidly to its equilibrium position. 

 To achieve this purpose, the fibre was connected through a 

 comparatively low resistance to earth. Suitable resistances 

 can be simply made by using capillary tubings filled with a 

 mixture of xylol and alcohol*. The back leak so provided 

 * Campbell, Phil. Mag. xxiii. p. 668 (1912). 



