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THE POPULAR SCIENCE MONTHLY 



when only a few alpha particles enter the detecting vessel per minute. 

 This difficulty can be got over by the use of a string electrometer in 



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Fig. 4. Photographic Effect due to Alpha Particles from a Central Point. 



which the moving system consists of a fine silvered quartz fiber sus- 

 pended between two charged parallel plates and viewed with a high- 

 power microscope. The entrance of an alpha particle is shown by a 

 sudden movement of the fiber, and if the current is allowed to leak away 

 through a suitable resistance, the fiber returns to the position of rest in 

 a small fraction of a second. The movement of the fiber can be re- 



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Fig. 5. Photographic Record on String Electrometer of Entrance of Alpha 

 Particles into the Detecting Vessel. 



corded photographically on a moving film, and it is possible in this 

 way to count accurately the number of particles, even if several thou- 

 sand enter the detecting vessel per minute. 



Examples of such photographic records, obtained by Eutherford 

 and Geiger, are shown in Fig. 5. The vertical movements of the fiber 

 from the horizontal line are due to the entrance of alpha particles, and 

 it is seen how clearly the detailed movements of the fiber are registered. 

 In some cases, one alpha particle follows another so rapidly that the 



