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The technique of using an outer pair of electrodes to inject current and an inner pair of electrodes to measure potential eliminates electrode/electrolyte impedance errors when measuring the resistivity of a conducting substance. A pressing problem of the late 1800s was the accurate measurement of the resistivity of an electrolyte that was placed between a pair of electrodes in a container. Kohlrausch (1897) working in Charlottenburg, Germany, solved the problem in a limited way by 1) using 1000 Hz current and 2) creating the platinum-black electrode. By using what was then a high frequency, the electrode impedance became low. By placing a velvety-like deposit of platinum on a platinum electrode, the effective surface area was increased, thereby reducing the electrode impedance. Such blackened platinum electrodes are still used in conductivity cells designed to permit measurement of electrolytic resistivity. Working in Paris, France, well before Kohlrausch, Lippmann had solved the problem of measuring electrolytic resistivity in a simple and elegant way by introducing the tetrapolar method in conjunction with his capillary electrometer, a sensitive and rapidly responding potential indicator. Lippmann (1873) had become interested in the shape of a drop of mercury in a solution of sulfuric acid. When an iron wire was placed into the sulfuric acid and then contacted the mercury, the contour of the mercury changed. The combination of the iron wire, mercury, and electrolyte caused the charge distribution on the surface of the mercury to change, thereby altering the surface tension and, hence, the contour. Lippmann used the phenomenon of electrocapillarity, as it became known, to construct a sensitive and rapidly responding electrometer.
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