Fig. 155. LIPPMAN'S CAPILLARY ELECTROMETER.
Electrometer, Capillary.
An electrometer for measuring potential difference by capillary action,
which latter is affected by electrostatic excitement. A tube A contains
mercury; its end drawn out to a fine aperture dips into a vessel B which
contains dilute sulphuric acid with mercury under it, as shown. Wires
running from the binding-posts a and b connect one with the mercury in
A, the other with that in B. The upper end of the tube A connects with a
thick rubber mercury reservoir T, and manometer H. The surface tension
of the mercury-acid film at the lower end of the tube A keeps all in
equilibrium. If now a potential difference is established between a and
b, as by connecting a battery thereto, the surface tension is increased
and the mercury rises in the tube B. By screwing down the compressing
clamp E, the mercury is brought back to its original position. The
microscope M is used to determine this position with accuracy. The
change in reading of the manometer gives the relation of change of
surface tension and therefore of potential. Each electrometer needs
special graduation or calibration, but is exceedingly sensitive and
accurate. It cannot be used for greater potential differences than .6
volt, but can measure .0006 volt. Its electrostatic capacity is so small
that it can indicate rapid changes. Another form indicates potential
difference by the movement of a drop of sulphuric acid in a horizontal
glass tube, otherwise filled with mercury, and whose ends lead into two
mercury cups or reservoirs. The pair of electrodes to be tested are
connected to the mercury vessels. The drop moves towards the negative
pole, and its movement for small potential differences (less than one
volt) is proportional to the electro-motive force or potential
difference.