. Radioactive transformations . method is shown in Fig. 43. The two upperFig. 43. bands A represent the traces of the pencil of raysRetardation of obtained by reversal of the magnetic field for the the Jtrticles °n ^^ fr°m the micovered half of the wire 5 thepassing through lower bands B were obtained for the rays frommatter. the wire, when covered with eight layers of alu- minium foil, each of thickness about .00031 apparatus was exhausted during the experiment, so thatthe absorption of the rays in air is negligible. The greater deflection of the pencil of rays which have passedthroug
. Radioactive transformations . method is shown in Fig. 43. The two upperFig. 43. bands A represent the traces of the pencil of raysRetardation of obtained by reversal of the magnetic field for the the Jtrticles °n ^^ fr°m the micovered half of the wire 5 thepassing through lower bands B were obtained for the rays frommatter. the wire, when covered with eight layers of alu- minium foil, each of thickness about .00031 apparatus was exhausted during the experiment, so thatthe absorption of the rays in air is negligible. The greater deflection of the pencil of rays which have passedthrough the aluminium is clearly seen in Fig. 43. It will beshown later that the value of e/m for the particles does notchange in consequence of their passage through matter. Thegreater deflection of the rays is then due to a decrease of theirvelocity after passing through the screen. This velocity isinversely proportional to the distance between the centres of thebands. 1 Rutherford : Phil. Mag., July, 1905 ; Jan. and April, PROPERTIES OF THE a RAYS 225 We have seen that the a particles from radium C are all pro-jected initially at the same speed. The absence of dispersionof the rays after passing through the screen shows that thevelocity of all the a particles is reduced by the same amount intraversing the screen. The following table gives the velocity of the a particles fromradium C after passing through successive layers of aluminiumfoil, each of thickness about .00030 cms. The velocity is ex-pressed in terms of F0, the velocity of the a particles fromradium C with an uncovered wire. Number of layers of Velocity of aluminium foil. a particles. 0 v0 2 .94 4 .87 6 .80 8 .72 10 .63 12 .53 14 .43 not measurable There is a marked weakening of the photographic effect of therays after passing through 10 layers of foil. The photographicimpression is weak, but distinct with 13 layers, and, using veryactive wires, can be observed with 14 layers. On account ofthis falling
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