=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. [Illustration: 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.