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 Dolezalek8. 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