Page:Outlines of Physical Chemistry - 1899.djvu/208

 ��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

�� �