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Dr. H. Geiffer and Prof. E". Rutherford 



on 



is ordinarily very much greater than can be used in counting 

 with the quadrant electrometer. This large back leak has 

 the great advantage of cutting out the great majority of 

 slow electrical disturbances, for only rapid changes of 

 potential make their effects visible. 



The type of cylindrical vessel used in our original investi- 

 gations did not fulfil condition (2) on account of scattering 

 of the a. particles by the mica-covered opening through 

 which they entered. On account of this scattering, the 

 a particles have different lengths of path in the gas, and 

 produce unequal throws. The differences in the throws 

 become less marked the smaller the length of the tube 

 compared with its diameter. To overcome this difficulty, a 

 detecting vessel of the type shown in fig. 1 was constructed. 



Fur. 1. 



It consisted of a metallic hemisphere, near the centre of 

 which was a spherical electrode A supported by a metal rod. 

 The a particles entered in the hemisphere through the 

 opening B which was covered with a thin sheet of mica. 

 Helium at suitable pressure was introduced into the apparatus. 

 With this arrangement, it was found that the throws were 

 remarkably uniform, so that it was easy to recognize by the 

 length of the throw when two or even three a particles 

 entered the vessel in rapid succession. 



In these experiments we have used helium at a pressure 

 of about one-third of an atmosphere as the gas in the 

 detecting vessel. The use of helium has many advantages. 

 In the first place, the potential difference required is much 

 lower than with air or carbon dioxide at the same pressure. 

 This potential was found to increase very much when small 

 impurities were present with the helium. As is well known, 

 however, the helium can easily be purified by passing it over 

 charcoal cooled by liquid air. This method of purification 

 of helium was used in all the experiments. Using helium 

 in the detecting vessel, the necessary magnification could 



