The metal vessel V containing a large amount of thoria is connected with the positive pole of a battery of about 300 volts. The wire AB to be made active is fastened to a stouter rod BC, passing through an ebonite cork inside a short cylinder D, fixed in the side of the vessel. This rod is connected with the negative pole of the battery. In this way the wire AB is the only conductor exposed in the field with a negative charge, and it is found that the whole of the excited activity is concentrated upon it.
In this way it is possible to make a short thin metal wire over 10,000 times as active per unit surface as the thoria from which the excited activity is derived. In the same way, the excited activity due to radium can be concentrated mainly on the negative 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