6 vol. water). The reservoir b contains dilute sulphuric acid, but has a layer of mercury in the bottom. Two platinum wires a and /3 (the latter being separated from the sulphuric acid by means of a glass tube) are used for completing the circuit in the apparatus or for putting it in the circuit of a current. When the apparatus is closed on itself (by a and /3 being joined) the mercury in the capillary tube takes up a position of equilibrium which is the zero of the apparatus. If the apparatus be now interposed in a circuit, there will be a potential-difference e between a and /3, and the meniscus of the mercury will be displaced and take up a new position of equilibrium. The length of capillary tube along which this displacement occurs depends on the value of e, and within narrow limits may be used as a measurement of the potential-difference.
N.B. — The displacement of the mercury is only a secondary phenomenon and is caused by a change which takes place in the surface tension of the metal. In order to maintain the meniscus of the mercury perfectly clean, a must only be connected with a negative pole, and the cathodic polarisation must never exceed 1 volt (otherwise hydrogen could be evolved at a).
The instrument is best read by means of a not very powerful microscope furnished with a micrometer-ocular. Lippmann's electrometer is, however, more often used for determining the equilibrium of potentials in compensation methods which we shall very shortly come to.
Measurement of Electromotive Force
There are certain direct methods relying on the use of a galvanometer or of an electrometer, but both for exact- ness in the results and for simplicity in working it is preferable to reduce the measurement of an electromotive force to the measurement of a resistance, as is done in Pogoendorff's Compensation Method.
In fig. 43, e represents a constant current element the
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