78 METHODS OF MEASUREMENT [cH. 
ordinary short glass jar partly filled with sulphuric acid is, in most 
cases, not easy to insulate to this extent. It is better to replace 
it by an ebonite (or sulphur) condenser! such as is shown in 
Fig. 14. 
A circular plate of ebonite about 1 em. thick is turned down 
until it is not more than $mm. 
thick in the centre. Into this 
circular recess a brass plate B fits 
loosely. The ebonite plate rests 
on another brass plate C connect- 
ed with earth. The condenser 
thus formed has a considerable 
capacity and retains a charge for 
along 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 connec- 
tion 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 atmo- 
sphere, 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. 1s available, it 1s much better to 
keep it constantly connected with the needle and to avoid the use 
of the condenser altogether. Ifa battery of small accumulators is 
used, their potential can always be kept at a constant value, and 
the electrometer always has a constant sensibility. 

Fig. 14. 
56. <A very useful electrometer of great sensibility has recently 
been devised by Dolezalek®. 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 
1 Strutt, Phil. Trans. A, p. 507, 1901. 
2 Instrumentenkunde, p. 345, Dec. 1901. 
