d

If the condensation chamber has two parallel plates for the
application of an electric field like that already described, the
ions will be carried at once to the electrodes and disappear. The
rapidity of this action depends upon the strength of the electric
field and experiment shows that the stronger the field the smaller the
number of condensation drops formed. If there is no electric field, a
cloud can be produced some time after the shutting off of the source
of radiation, showing that time is required for the recombination of
the ions.

Size and Nature of Ions

If the drops are counted (there being special methods for this) and
the total current carried accurately measured, then the charge carried
by each ion may be calculated. This has been determined. The mass of
an ion compared with the mass of the molecules of gas in which it was
produced can also be approximately estimated. In the study of these
ions the view has been held that the charged ion attracted to itself a
cluster of molecules which surrounded the charged nucleus and traveled
with it. It is roughly estimated that about thirty molecules of the
gas cluster around each charged ion.

Photographing the Track of the Ray

Utilizing the fact that these ions with their clusters of molecules
form nuclei for the condensation of water vapor, C. T. R. Wilson has
by instantaneous photography been able to photograph the track of an
ionizing ray through air. The number of the ions produced, and hence
the number of drops, is so great that the trail is shown as a
continuous line. In the copy of this photograph it will be seen that
at some distance from its source the straight trail is slightly but
abruptly bent. Near the end of its course there is another abrupt and
much sharper bend. These bends show where the ionizing ray, in this
case an alpha particle, has been deflected by more or less direct
collision with an atom. These collisions and the final disappearance
of the ray will be discussed later.

[Illustration: FIG. 2.--PHOTOGRAPH OF THE TRACK OF AN
IONIZING RAY.]

Action of Radiations on Photographic Plates

Taking up now other means of examining these radiations, it is well to
consider their action upon a photographic or sensitive plate. It will
be recalled that this was the method by which their existence was
originally detected. To illustrate the method, the following account
of how one such photograph was taken may be given.

The