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 detector for counting alpha particles. The arrangement, which is very simple, is shown in Fig. 6. A fine sharply pointed needle ends about
Fig. 6. Geiger's Detector of Individual Alpha and Beta Particles.
one 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 deflection of the fiber. So sensitive is this method, that Geiger has found that individual beta particles can easily be detected and counted by its aid. This is very remarkable when it is remembered that the ionization effect
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-
