Apparatus for the determination aboard ship of the salinity of sea water by the electrical conductivity method

The apparatus described furnishes a convenient means for the determination of the salinity of samples of sea water as they may be collected. It consists essentially of a Wheatstone bridge, conductivity cells of a type which may be filled and emptied during fairly rough weather, and the necessary auxiliary equipment, all built into a single cabinet. A special feature is the use of two similar conductivity cells in adjacent arms of the bridge. When both are filled with samples of sea water, even though not of the same salinity, both have very nearly the same temperature coefficient, so the balance of the bridge is not greatly affected by small uncertainties of the temperature. Further, the complicated actions taking place at the electrodes, generally referred to as polarization, are substantially the same in both cells, so the effects of polarization are largely neutralized. A further special feature, not really a part of the equipment but of a plan for its use, is that a supply of standard sea water or sea water of which the salinity has been determined by laboratory methods is carried aboard and used for periodic standardizations. Samples of sea water to be tested, together with a sample of standard sea water, are substituted one after another in the same conductivity cell. This gives a direct comparison of the sample of unknown salinity with that of a standard, almost if not quite independent of the cell constant, largely independent of other constants of the equipment and largely independent of systematic errors. Apparatus of the type has been used during the past several seasons in the International Ice Patrol Service, also on the Marion expedition and on the last cruise of the nonmagnetic vessel Carnegie.