Radio-Activity

by Ernest Rutherford

Fig. 15.

A circular plate of ebonite about 1 cm. thick is turned down until it is not more than ½ mm. thick in the centre. Into this circular recess a brass plate B fits loosely. The ebonite plate rests on another brass plate C connected with earth. The condenser thus formed has a considerable capacity and retains a charge for a long time. In order to make connection with the needle, a small glass vessel D, partly filled with sulphuric acid, is placed on the plate B and put in connection with the needle by means of a fine platinum wire. The platinum wire from the needle dips into the acid, and serves to damp the needle. In a dry atmosphere, a condenser of this kind will not lose more than 20 per cent. of its charge in a week. If the insulation deteriorates, it can readily be made good by rubbing the edge of the ebonite A with sand-paper, or removing its surface in a lathe.

If a sufficient and steady E.M.F. is available, it is much better to keep the battery constantly connected with the needle, and to avoid the use of the condenser altogether. If a battery of small accumulators is used, their potential can be kept at a constant value, and the electrometer always has a constant sensibility.

61. A very useful electrometer of great sensibility has been devised by Dolezalek[^[106]]. It is of the ordinary quadrant type with a very light needle of silvered paper, spindle shaped, which lies fairly close to the quadrants. A very fine quartz suspension is employed. In consequence of the lightness of the needle and its nearness to the quadrants, it acts as its own damper. This is a great advantage, for difficulties always arise when the wire dips into sulphuric acid, on account of the thin film which collects after some time on the surface of the acid. This film obstructs the motion of the platinum wire dipping into the acid, and has to be 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.