og| METHODS OF MEASUREMENT Hr 
is to a large extent avoided in the White pattern of the Kelvin 
electrometer, which is provided with a replenisher and attracted 
disc for keeping the potential of the needle at a definite value. If 
sufficient trouble is taken im insulating and setting up this type 
of electrometer, it proves a very useful instrument of moderate 
sensibility, and will continue in good working order for a year or 
more without much attention. 
Simpler types of electrometer of greater sensibility can however 
be readily constructed to give accurate results. The old type of 
quadrant electrometer, to be found in every laboratory, can readily 
be modified to prove a useful and trustworthy instrument. A light 
needle can be simply made of thin aluminium, of silvered paper or 
of a thin plate of mica, covered with gold-leaf to make it conducting. 
The aluminium wire and mirror attached should be made as light 
as possible. The needle should be suspended either by a fine 
quartz fibre or a long bifilar suspension of silk. A very fine 
phosphor bronze wire of some length is also very satisfactory. 
A magnetic control is not very suitable, as it is disturbed by coils 
or dynamos working in the neighbourhood. In addition, the zero 
point of the needle is not as steady as with the quartz or bifilar 
suspension. 
When an electrometer is used to measure a current by noting 
the rate of movement of the needle, it is essential that the needle 
should be damped sufficiently to give a uniform motion of the spot 
of light over the scale. The damping requires fairly accurate 
adjustment. If it is too little, the needle has an oscillatory move- 
ment superimposed on the steady motion; if it 1s too great, it 
moves too sluggishly from rest and takes some time to attain 
a state of uniform motion. With a light needle, very little, if any, 
extra damping is required. A light platinum wire with a single 
loop dipping in sulphuric acid is generally sufficient for the purpose. 
With light needle systems and delicate suspensions, it is only 
necessary to charge the needle to a potential of a few hundred volts 
to give a sensibility of several thousand divisions for a volt. With 
such low potentials, the difficulty of insulation of the condenser, 
with which the needle is in electrical connection, is much reduced. 
It is convenient to use a condenser such that the potential of the 
needle does not fall more than a few per cent. per day. The 
