Fig. 62.
Plates A, B, C, D, E are placed in various positions in the enclosure. After exposure for a day, the plates after removal are found to be radio-active even in positions completely shielded from the action of the direct rays. For example, the plate D shielded from the direct radiation by the lead plate P is as active as the plate E, exposed to the direct radiation. The amount of activity produced in a given time on a plate of given area in a definite position is independent of the material of the plate. Plates of mica, copper, cardboard, ebonite, all show equal amounts of activity. The amount of activity depends on the area of the plate and on the amount of free space in its neighbourhood. Excited radio-activity is also produced in water if exposed to the action of an emanating compound.
176. Concentration of excited radio-activity on the negative electrode. When thorium or radium is placed in a closed vessel, the whole interior surface becomes strongly active. In a strong electric field, on the other hand, the writer found that the activity was confined entirely to the negative electrode. By suitable arrangements, the whole of the excited activity, which was previously distributed over the surface of the vessel, can be concentrated on a small negative electrode placed inside the vessel. An experimental arrangement for this purpose is shown in [Fig. 63].
Fig. 63.
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 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.