ceases. In this manner the range is, in fact, determined. The
dotted curve (Fig. 13) depicts the recent investigation of the
ionisation effected by a sheaf of parallel rays of radium C in
air, as determined by Geiger. The range is laid out horizontally
in centimetres. The numbers of ions are laid out vertically. The
remarkable nature of the results will be at once apparent. We
should have expected that the ray at the beginning of its path,
when its velocity and kinetic energy were greatest, would have
been more effective than towards the end of its range
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when its energy had almost run out. But the curve shows that it
is just the other way. The lagging ray, about to resign its
ionising properties, becomes a much more efficient ioniser than
it was at first. The maximum efficiency is, however, in the case
of a bundle of parallel rays, not quite at the end of the range,
but about half a centimetre from it. The increase to the maximum
is rapid, the fall from the maximum to nothing is much more
rapid.
It can be shown that the ionisation effected anywhere along the
path of the ray is inversely proportional to the velocity of the
ray at that point. But this evidently does not apply to the last
5 or 10 mms. of the range where the rate of ionisation and of the
speed of the ray change most rapidly. To what are the changing
properties of the rays near the end of their path to be ascribed?
It is only recently that this matter has been elucidated.
When the alpha ray has sufficiently slowed down, its power of
passing right through atoms, without appreciably experiencing any
effects from them, diminishes. The opposing atoms begin to exert
an influence on the path of the ray, deflecting it a little. The
heavier atoms will deflect it most. This effect has been very
successfully investigated by Geiger. It is known as "scattering."
The angle of scattering increases rapidly with the decrease of
velocity. Now the effect of the scattering will be to cause some
of the rays to complete their ranges
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or, more accurately, to leave their direct line of advance a
little sooner than others. In the beautiful experiments of C. T.
R. Wilson we are enabled to obtain ocular demonstration of the
scattering. The photograph (Fig. 14.), which I owe to the
kindness of Mr. Wilson, shows the deflection of the ray towards
the end of its path. In
{Fig. 14}
this case the path of the ray has been rendered visible by the
condensation of water particles under the influence of the
ionisation; the atmosphere in which the ray travels being in a
state of supersaturation with water vapour at the instant of the
passage of the ray. It is evident that if we were observing the
ionisation along a sheaf of parallel rays, all starting with
equal velocity,
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