An $\mathrm{\alpha }-\mathrm{particle}$ and a proton are both simultaneously projected in opposite directions into a region of constant magnetic field perpendicular to the direction of the field. After some time it is found that the velocity of the $\mathrm{\alpha }-\mathrm{particle}$ has changed in direction by 45°. Then at this time, the angle between velocity vectors of $\mathrm{\alpha }-\mathrm{particle}$ and proton is