180 



ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1915. 



viewed with a high-power microscope. The entrance of an alpha 

 particle is shown by a sudden movement of the fiber, and if the cur- 

 rent 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 recorded photographically on a moving 

 film, and it is possible in this way to count accurately the number 

 of particles, even if several thousand enter the detecting vessel per 

 minute. 



Examples of such photogi'aphic records, obtained by Rutherford 

 and Geiger, are shown in plate 2, figure 2. The vertical movements of 

 the fiber from the horizontal line are due to the entrance of alpha par- 

 ticles, 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 fiber has not time to come to rest in between, and 

 this is shown by the sawlike appearance of some of the peaks in the 

 photograph. It will be noticed also that while the heights of most 



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Fig. 3. — Geiger's detector of individual alpha and beta particles. 



of the deflections are nearl}^ 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 ir- 

 regularly 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 figure 3. A fine sharply pointed needle ends 

 about 1 centimeter from the opening O, 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 deflec- 

 tion of the fiber. So sensitive is this method that Geiger has found 

 that individual beta particles can easily be detected and counted by 



