electrode. In the case of thorium, if the central wire be charged positively, it shows no appreciable activity. With radium, however, a positively charged body becomes slightly active. In most cases, the amount of activity produced on the positive electrode is not more than 5% of the corresponding amount when the body is negatively charged. For both thorium and radium, the amount of excited activity on electrodes of the same size is independent of their material.
All metals are made active to equal extents for equal times of exposure. When no electric field is acting, the same amount of activity is produced on insulators like mica and glass as on conductors of equal dimensions.
177. Connection between the emanations and excited activity. An examination of the conditions under which excited
activity is produced shows that there is a very close connection
between the emanation and the excited activity. If a thorium
compound is covered with several sheets of paper, which cut off the
α rays but allow the emanation to pass through, excited activity is
still produced in the space above it. If a thin sheet of mica is
waxed down over the active material, thus preventing the escape of
the emanation, no excited activity is produced outside it. Uranium
and polonium which do not give off an emanation are not able to
produce excited activity on bodies. Not only is the presence of
the emanation necessary to cause excited activity, but the amount
of excited activity is always proportional to the amount of emanation
present. For example, de-emanated thoria produces very
little excited activity compared with ordinary thoria. In all cases
the amount of excited activity produced is proportional to the
emanating power. When passing through an electric field the
emanation loses its property of exciting activity at the same
rate as the radiating power diminishes. This was shown by the
following experiment.
A slow constant current of air from a gasometer, freed from dust by its passage through cotton-wool, passed through a rectangular wooden tube 70 cms. long. Four equal insulated metal plates A, B, C, D, were placed at regular intervals along the tube. The positive pole of a battery of 300 volts was connected with a metal
