Ix] EXCITED RADIO-ACTIVITY 253 
electrode. In the case of thorium, if the central wire is 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. 
167. 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 
a rays but allow the emanation to pass through, excited activity 1s 
still produced in the space above it. Ifa thin sheet of mica is 
waxed down over the active material, thus preventing the escape of 
the emanation, no excited activity 1s 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 1s always proportional to the amount of emana- 
tion 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. ‘The emanation when passing through an 
electric field 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 rectangu- 
lar 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 to a metal 
