Page:Radio-activity.djvu/569

 The velocity of the α particles from the different radio-active products can at once be calculated, knowing the maximum range in air of the α rays from each product. The latter have been experimentally determined by Bragg. The velocity is expressed in terms of V_{0}, the initial velocity of the α particles from radium C. The rays from radium C are projected with a greater velocity than the rays from the other products of radium.

Product      Maximum range of           Velocity of           α particles in air    α particles

Radium         3 cms. ·82 V_{0} Emanation      3·8 or 4·4 cms. ·87 or ·90 V_{0} Rad. A         4·4 or 3·8  "         ·90 or ·87 V_{0} Rad. C          6·7         "        1·00 V_{0}

It is difficult to determine from the experiments whether the range 3·8 cms. belongs to the rays from the emanation or from radium A. The mean velocity of the α particles is thus ·90 V_{0}, and the maximum variation for the individual products does not vary more than 10 per cent. from the mean value.

The results of Becquerel, discussed in section 92, at once receive an explanation on the above results. The α particles, expelled from radium in radio-active equilibrium, have all ranges lying between 0 and 6·7 cms. of air. The velocity of the α particles which are able to produce a photographic impression varies between ·64 V_{0} and V_{0}. The particles which have only a short range in air are projected with a smaller velocity than those which have a greater range. The former are in consequence more bent by a magnetic field. It is thus to be expected that the apparent curvature of the path of rays in a uniform magnetic field will be greater close to the radium than at some distance away.

Range of phosphorescent action in air. Some experiments were also made to see whether the action of the α rays in producing luminosity in substances like zinc sulphide, barium platino-cyanide, and willemite, ceased at the same distance as the ionizing action.

A very active wire was placed on a moveable plate, the distance of which from a fixed screen of phosphorescent substance could be varied. The distance at which the phosphorescent action ceased could be determined fairly accurately. Different thicknesses of aluminium foil were then placed over the active wire, and the corresponding distance at which the luminosity disappeared was