. Radioactive transformations . l meas-urement, Becquerel found that these two diverg-ing lines were not accurately the arcs of a circle,but that the radius of curvature of the path ofthe rays increased with the distance from thesource. Becquerel thought that the a rays fromradium were homogeneous, and concluded fromthis experiment that the value of e/m of theparticles progressively decreased in their passagethrough air, in consequence of an increase of m by accretionsfrom the air. Bragg,2 however, showed that this peculiarity in the trace ofthe rays could be simply explained without any assum
. Radioactive transformations . l meas-urement, Becquerel found that these two diverg-ing lines were not accurately the arcs of a circle,but that the radius of curvature of the path ofthe rays increased with the distance from thesource. Becquerel thought that the a rays fromradium were homogeneous, and concluded fromthis experiment that the value of e/m of theparticles progressively decreased in their passagethrough air, in consequence of an increase of m by accretionsfrom the air. Bragg,2 however, showed that this peculiarity in the trace ofthe rays could be simply explained without any assumption ofan alteration in the value of e/m,, by taking into account the com-plexity of the pencil of rays. The experimental arrangementis diagrammatically shown in Fig. 46. S P and S P representthe diverging traces of the rays on the photographic plate in auniform magnetic field after emerging from the slit S. Let us 1 Becquerel: Comptes rendus, cxxxvi, pp. 199, 431, 977, 1517 (1903). 2 Bragg: Phil. Mag., Dec, 1904; April, Fig. 46. 234 RADIOACTIVE TRANSFORMATIONS consider, for example, the outside edge of the trace at a point photographic effect at this edge of the trace is due to theparticles of lowest velocity, which are just able to produce pho-tographic action at A. Consider next a point B further removedfrom the source. The a particles, which produce the edge ofthe trace, have the same velocity as in the first case; but sincethey have had to travel through a distance B R of air instead ofA R, they must have initially started with a greater must evidently be the case, since the a particle is retardedin its passage through air. The average velocity of these a par-ticles along their path is consequently greater than in the firstcase, and the outside edge will be deflected through a smaller dis-tance than would be expected if the average velocity were thesame for the two paths A R, B R. This will cause the trace ofthe rays to show evidence of steadily
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