THE CONSTITUTION OF MATTER 



119 



fiber has not time to come to rest in between, and this is shown by the 

 saw-like appearance of some of the peaks in the photograph. It will 

 be noticed also that while the heights of most of the deflections are 

 nearly the same, in a few cases the deflections are nearly twice as great 

 as the normal. This is due to the nearly simultaneous entrance of two 

 alpha particles into the vessel. Although the photographic film moved 

 at a constant rate, it is seen that the throws due to the alpha particles 

 are distributed very irregularly along it. A close examination of such 

 records shows that variations of this kind are in accord with the ordinary 

 laws of probability. 



During this year, Dr. Geiger has found a still more sensitive de- 

 tector for counting alpha particles. The arrangement, which is very 

 simple, is shown in Fig. 6. A fine sharply pointed needle ends about 



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Fig. 6. Geiger's Detector of Individual Alpha and Beta . Particles. 



one centimeter from the opening 0, where the alpha particles enter. 

 If the outer brass tube be charged positively to about 1,000 volts, and 

 the needle connected with a string electrometer, it is found that the 

 entrance of an alpha particle produces a very great deflection of the 

 fiber. So sensitive is this method, that Geiger has found that indi- 

 vidual beta particles can easily be detected and counted by its aid. 

 This is very remarkable when it is remembered that the ionization effect 



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Fig. 7. 



Record with String Electrometer, Upper Record for Beta Particles. 

 Lower for Alpha Particles. 



due to a beta particle is on the average not more than 1/100 of that 

 due to an alpha particle. 



A photographic record of the entrance of beta particles into the 

 detecting vessel is shown in Fig. 7. The upper record is for beta par- 



