Relative velocities of an alpha particle and an electron which are favorable for the capture of the electron.—A stream of electrons from a thermionic source is superposed on a beam of $\alpha$-particles from polonium. To determine the number of captures, the $\alpha$-particle beam is subjected to a magnetic field and the number of deflected $\alpha$-particles counted by the scintillation method. If an $\alpha$-particle captures an electron before entering the magnetic field it will no longer be deflected to so great an extent. The number of scintillations for various velocities of the electrons, both greater and less than that of the $\alpha$-particles, was observed. It was found that captures take place only at definite electron velocities. These velocities are related to velocities in the Bohr orbits of ionized helium as follows: $v=u-w$ and $v=w^{\prime }-u$ where $u$ is the velocity of the $\alpha$-particle, $w$ and $w^{\prime }$ are two velocities of the electron at which capture takes place and $v$ is the velocity of an electron in any one of the circular Bohr orbits. A plot of the number of captures as a function of the voltage impressed on the electron stream gives a series of maxima on each side of the equivalent voltvelocity of the $\alpha$-particles which correspond to the energy levels of ionized helium.

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