. Radio-activity . ture of the path of rays in a uniformmagnetic field will be greater close to the radium than at somedistance away. Range of phosphorescent action in air. Some experimentswere also made to see whether the action of the a rays in producingluminosity 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 distanceof which from a fixed screen of phosphorescent substance could bevaiied. The distance at which the phosphorescent action ceasedcould be determined
. Radio-activity . ture of the path of rays in a uniformmagnetic field will be greater close to the radium than at somedistance away. Range of phosphorescent action in air. Some experimentswere also made to see whether the action of the a rays in producingluminosity 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 distanceof which from a fixed screen of phosphorescent substance could bevaiied. The distance at which the phosphorescent action ceasedcould be determined fairly accurately. Different thicknesses ofaluminium foil were then placed over the active wire, and thecorresponding distance at which the luminosity disappeared was 35—2 548 APPENDIX A measured. The results are shown graphically in Fig. 107, where theordinates represent the distance of the phosphorescent screen fromthe active wire, and the abscissae the number of layers of aluminiumfoil, each 00031 cms. 4 6 8 10Layers of A luminlum Foil Fig. 107. It is seen that the curve joining the points is a straight thicknesses of foil absorbed the rays to the same extent as 6 8 air, so that each thickness of aluminium corresponded in absorbingpower to -54 cms. of air. For a screen of zinc sulphide, the phospho-rescent action ceased at a distance of air of 68 cms., showing that thephotographic and phosphorescent ranges of the a rays in air werepractically identical. The experiments with barium platino-cyanide and willemite weremore difficult, as the /3 and y rays from the active wire produceda luminosity comparable with that produced by the a rays. Fairlyconcordant results, however, were obtained by introducing a thinsheet of black paper between the active wire and the screen. If theluminosity was sensibly changed, it was concluded that the a raysstill produced an effect, and in this way the point of cessation ofphosphorescent action could be approximately det
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