If an apparatus is to be tested at some distance from the electrometer, the wires leading to it should be insulated in metal cylinders connected with earth. The size of the insulators used at various points should be made as small as possible, in order to avoid disturbances due to their electrification. In damp climates, paraffin, amber, or sulphur insulates better than ebonite. The objection to paraffin as an insulator for sensitive electrometers lies in the difficulty of getting entirely rid of any electrification on its surface. When paraffin has been once charged, the residual charge, after diselectrifying it with a flame, continues to leak out for a long interval. All insulators should be diselectrified by means of a spirit-lamp or still better by leaving some uranium near them. Care should be taken not to touch the insulation when once diselectrified.
In accurate work it is advisable to avoid the use of gas jets or Bunsen flames in the neighbourhood of the electrometer, as the flame gases are strongly ionized and take some time to lose their conductivity. If radio-active substances are present in the room, it is necessary to enclose the wires leading to the electrometer in fairly narrow tubes, connected with earth. If this is not done, it will be found that the needle does not move at a constant rate, but rapidly approaches a steady deflection where the rate of loss of charge of the electrometer and connections, due to the ionization of the air around them, is balanced by the current to be measured. This precaution must always be taken when observations are made on the very penetrating rays from active substances. These rays readily pass through ordinary screens, and ionize the air around the electrometer and connecting wires. For this reason it is impossible to make accurate measurements of small currents in a room which is used for the preparation of radio-active material. In course of time the walls of the room become radio-active owing to the dissemination of dust and the action of the radio-active emanations[1].
- ↑ It is very desirable that care should be taken not to release large quantities of the radium emanation inside a laboratory. This emanation has a slow rate of decay and is carried by currents of air throughout the whole building and finally leaves behind an active deposit of very slow rate of change (see chapter XI.). Eve (Nature, March 16, 1905) has drawn attention to the difficulty of making refined radio-active measurements under such conditions.
