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[1]. 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
- ↑ Dolezalek, Instrumentenkunde, p. 345, Dec. 1901.
