Action of the Magnetic Field upon the α-Rays.
In a recent work, Mr. Rutherford announced that, in a powerful electric or magnetic field, the α-rays of radium are slightly deflected, in the manner of particles positively electrified and possessing great velocity. Mr. Rutherford concludes from his experiments that the velocity of the α-rays is of the order of magnitude 2·5 × 109c.m.
sec. and that the ratio e
m for these rays is of the order of magnitude 6 × 103, which is 104 times as great as for the deflected β-rays. We shall see later that these conclusions of Mr. Rutherford are in agreement with the properties already known of the α-radiation, and that they account, in part at least, for the law of absorption of this radiation.
The experiments of Mr. Rutherford have been confirmed by M. Becquerel. M. Becquerel has further demonstrated that polonium rays behave in a magnetic field like the α-rays of radium, and that, for the same field, they seem to have the same curvature as the latter.
It also appears from M. Becquerel’s experiments that the α-rays do not form a magnetic spectrum, but act rather like a homogeneous radiation, all the rays being equally deflected.
Action of the Magnetic Field on the Rays of other Radio-active Substances.
We have just seen that radium gives off α-rays comparable to the tube rays, β-rays comparable to cathode rays, and γ-rays which are penetrating and not deflected. Polonium gives off α-rays only. Amongst the other radio-active substances, actinium seems to behave like radium, but the study of its radiation has not yet advanced so far as in the case of radium. As regards the faintly radio-active bodies, we know to-day that uranium and thorium give rise to α-rays as well as β-rays (Becquerel, Rutherford).
Proportion of β-Rays in the Radiation of Radium.
As I have already mentioned, the proportion of β-rays increases with increase of distance from the source of radiation. These rays never occur alone, and for great distances the presence of γ-rays is always discernible. The presence of very penetrating, undeflected rays in the radiation of radium was first observed by M. Villard. These rays constitute only a small portion of the radiation measured by the electrical method, and their presence escaped our notice in our first experiments, so that we believed falsely that the radiation at great distances contained only rays capable of deflection.
The following are the numerical results obtained with experiments made by the electrical method with an apparatus similar to that of Fig. 5. The radium was only separated from the condenser by the surrounding air. I shall indicate by the letter d the distance from the source of radiation to the condenser. The numbers of the second line represent the current subsisting when the magnetic field is acting, supposing the current obtained with no field equal to 100 for each distance. These numbers may be considered as giving the percentage of the total α- and γ-rays, the deflection of the α-rays having been scarcely observable with the conditions employed.
At great distances there are no α-rays, and the undeflected radiation is therefore of the γ kind only.