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
which the moving system consists of a fine silvered quartz fiber suspended 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 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 photographic records, obtained by Rutherford 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