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THE POPULAR SCIENCE MONTHLY
A radioactive substance like radium emits not only alpha particles but beta particles which are electrons in very swift motion. These beta particles are generally far more penetrating than the alpha rays, but produce a much smaller number of ions per centimeter of their path
Fig. 9. Magnified Track of Alpha Particles (Wilson).
through a gas. In Fig. 10 is seen the track of a swift beta particle crossing the expansion chamber. It will be observed that the path is not straight but tortuous, due to the marked scattering of the particle by collisions with the atoms of matter in its path. Although the trail
Fig. 10. Tracks of Beta Particles.
