ut] METHODS OF MEASUREMENT 79 
employed. In consequence of the lightness of the needle and the 
nearness to the quadrants it acts as its own damper. This is 
a great advantage, for difficulties always arise with the wire dipping 
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 be readily 
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 electro- 
static units) but the imcreased 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. The quartz 
fibre can be made sufficiently conducting for this purpose by 
dipping it into a dilute solution of calcium chloride. In addition 
to its great sensibility, the advantages of this instrument lie in the 
steadiness of the zero and in the self-damping. 
57. Adjustment and screening. In adjusting an electro- 
meter, it is important to arrange that the needle lies 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 imsulation 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 
