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Rutherford discovered that at least two components are present in the radioactive radiations: alpha particles, which penetrate into aluminum only a few thousandths of a centimeter, and beta particles, which are nearly 100 times more penetrating. Subsequent experiments in which radioactive radiations were subjected to magnetic and electric fields revealed the presence of a third component, gamma rays, which were found to be much more penetrating than beta particles. In an electric field the path of the beta particles is greatly deflected toward the positive electric pole, that of the alpha particles to a lesser extent toward the negative pole, and gamma rays are not deflected at all. Therefore, the beta particles are negatively charged, the alpha particles are positively charged and are heavier than beta particles, and the gamma rays are uncharged.
The discovery that radium decayed to produce radon proved conclusively that radioactive decay is accompanied by a change in the chemical nature of the decaying element. Experiments on the deflection of alpha particles in an electric field showed that the ratio of electric charge to mass of these particles is about twice that of the hydrogen ion. Physicists supposed that the particles could be doubly charged ions of helium (helium atoms with two electrons removed). This supposition was proved by Rutherford when he allowed an alpha-emitting substance to decay near an evacuated thin-glass vessel. The alpha particles were able to penetrate the glass and were then trapped in the vessel, and within a few days the presence of elemental helium was demonstrated by use of a spectroscope. Beta particles were subsequently shown to be electrons, and gamma rays to consist of electromagnetic radiation of the same nature as X rays but of considerably greater energy.