��OUTLINES OF PHYSICAL CHEMISTRY
��increased by magnifying the movements of the needle. For this purpose a long light glass needle, blackened at its extremities, is fixed on to the astatic system. For delicate measurements a mirror galvanometer should be
���i
��used; the construction and methods of working of this
can be found in any text-book on physics.
Lippmann's Mercury Electrometer is one of the
most useful instruments. It may be graduated and used
for directly determining potential-differences (up to a maximum of 0*1 volt). It is generally employed to show equilibrium of potentials. Ostwald has devised the two adaptations of it shown in figs. 40 and 41 for electro- chemical work.
The instrument consists of the vertical tube a which connects with the reservoir b by means of a narrow tube,
whose internal bore must not be greater than 0*5 mm.
The tube a contains a column of mercury which by its
weight penetrates into the narrow tube and there meets a
capillary column of dilute sulphuric acid (1 vol. acid and
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