
80 METHODS OF MEASUREMENT [oH. 
insulation may be considered quite satisfactory. When a suitable 
desiccator is placed inside the tight-fitting electrometer case, the 
insulation of the quadrants should remain good for months. If the 
insulation of the ebonite deteriorates, 1t can easily be made good 
by removing the surface of the ebonite in a lathe. 
In working with a sensitive instrument lke the Dolezalek 
electrometer, it is essential that the electrometer and the testing 
apparatus should be completely enclosed in a screen of wire-gauze 
connected with earth, in order to avoid electrostatic disturbances. 
If an apparatus is to be tested at some distance from the electro- 
meter, the wires leading to it should be msulated in metal cylinders 
connected to earth. The size of the insulators used at various 
pots should be made as small as possible in order to avoid 
disturbances due to their electrification. In damp climates, paraffin 
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 
once paraffin has been charged, the residual charge, after dis- 
electrifying 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. 
