removed at regular intervals. These instruments can readily be made to give a sensibility of several thousand divisions for a volt when the needle is charged to about one hundred volts. The sensibility of the electrometer passes through a maximum as the potential of the needle is increased. It is always advisable to charge the needle to about the value of this critical potential. The capacity of the electrometer is in general high (about 50 electrostatic units) but the increased sensibility more than compensates for this. The needle may either be charged by lightly touching it with one terminal of a battery, or it may be kept charged to a constant potential through the quartz suspension.
Dolezalek states that the fibre can be made sufficiently conducting for the purpose by dipping it into a dilute solution of calcium chloride or phosphoric acid. I have not found this method satisfactory in dry climates as in many cases the fibre practically loses its conductivity after a few days exposure to dry air.
In addition to its great sensibility, the advantage of this instrument is in the steadiness of the zero and in the self-damping.
A sensibility of 10,000 millimetre divisions per volt can be readily obtained with this electrometer, if a very fine fibre be used. The use of such high sensibilities cannot, however, be recommended except for very special experiments. The period of swing of the needle under these conditions is several minutes and the natural leak of the testing vessels employed, as well as electrostatic and other disturbances, make themselves only too manifest. If measurements of minute currents are required, an electroscope of the type described in Section 56 is much to be preferred to a very sensitive electrometer. The electroscope readings in such a case are more accurate than similar measurements made by an electrometer.
For most measurements in radio-activity, an electrometer which has a sensibility of 100 divisions per volt is very suitable, and no advantage is gained by using an electrometer of greater sensibility. If still smaller effects require to be measured, the sensibility may be increased to several thousand divisions per volt.
62. Adjustment and screening. In adjusting an electrometer, it is important to arrange that the needle shall lie symmetrically with regard to the quadrants. This is best tested by observing whether the needle is deflected on charging, the quadrants all being earthed. In most electrometers there is an adjustable quadrant, the position of which may be altered until the needle is not displaced on charging. When this condition is fulfilled, the zero reading of the electrometer remains unaltered as the needle loses its charge, and the deflection on both sides of the zero should be the same for equal and opposite quantities of electricity.
The supports of the quadrants require to be well insulated. Ebonite rods are as a rule more satisfactory for this purpose than glass. In testing for the insulation of the quadrants and the connections attached, the system is charged to give a deflection of about 200 scale divisions. If the needle does not move more than one or two divisions after standing for one minute, the 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, it can easily be made good by removing the surface of the ebonite in a lathe.
In working with a sensitive instrument like 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 electrometer, the wires leading to it should be insulated in metal cylinders connected with earth. The size of