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European Agents: William Wesley & Son, 28 Essex St., Strand, London, and Mayer and Müller, Prins Louis-Ferdinand Str., Berlin. Exchanges.-The Journal does not exchange with other publications. Missing Numbers will be replaced without charge, provided that claim is made within thirty days after date of the following issue. Volume I, however, from July 19, 1911, to December 19, 1911, will be sent for $3.00. Special rates are given to members of scientifs societies affiliated with the Academy. THE WAVERLY PRESS 1 OCEANOGRAPHY.-An electrical instrument for recording seawater salinity. ERNEST E. WEIBEL and ALBERT L. THURAS, Bureau of Standards. (Communicated by S. W. Stratton.) The modern tendency in physical research is to replace indicating instruments by recording instruments wherever possible. This has been especially true in the science of meteorology, where the recent advances have been brought about almost entirely by the remarkable improvements and developments in recording instruments. In the related science of oceanography there are practically no recording instruments now in general use, except possibly the tide-gage. If meteorology has been so greatly benefited by such instruments, surely in oceanography, where the changes in the physical properties are so much more regular and therefore more easily interpreted, great advances should be looked for through the addition or substitution of recording instruments. A few years ago a recorder using a platinum resistance thermometer, giving a continuous record of the surface temperature of the ocean, was designed and constructed at the Bureau of Standards. This instrument has been used successfully on board ship and some very interesting records have been obtained which show the distribution of temperature and thereby indicate the location of ocean currents and also give a knowledge 1Done under the auspices of the Interdepartmental Committee on Oceanography, subcommittee on instruments, apparatus, and measurements. 2WAIDNER, DICKINSON, and CROWE. Bureau of Standards Bull. 10: 267. 1914. of their boundary conditions which could hardly be obtained by repeated single measurements of temperature. The temperature, however, is not nearly so reliable a clue to the location of currents and the origin of water masses as is the salinity. A body of sea water may change considerably in temperature in moving from one place to another, but unless the evaporation or rainfall is excessive its concentration will change comparatively little. By salinity is meant the number of grams of salt or solids in one kilogram of sea water. The compositionof these salts is very nearly constant everywhere in the open ocean, but the salinity, or concentration of the total salts, varies from place to place. From a consideration of the properties of sea water that vary with the salinity, the electrical conductivity seemed to be the most susceptible to continuous measurement, if the difficulty due to the variation of conductivity with temperature can be overcome. This difficulty is avoided by the use of a method which is compensated for temperature. This paper describes the method of measurement and the experimental work done towards the production of an apparatus to give a continuous record of sea-water salinity to the accuracy required in the most precise oceanographic research. The work has not been finished, but from the results obtained we believe that the method is practical and sufficiently important to warrant publication at the present time, even though the apparatus is not yet built. DESCRIPTION OF THE METHOD The method consists in measuring the ratio of the resistance of sea water in two equal or nearly equal electrolytic cells A and B (fig. 1); one cell A is sealed and contains sea water of a known average salinity, the other cell B is open and has flowing through it the sea water to be measured. This ratio is obtained by a Wheatstone bridge using alternating current to eliminate polarization effects in the cells. A calibration of the apparatus can be made at any time by using sea water of known salinity in the open cell. This can be done either by carrying standard samples or by determining the salinity of the sea water flowing through the open cell by some accurate method that can be used on board ship. Such a method giving salinity by a measurement of density has been described. A record of the resistance ratio of the two cells is made by a recorder similar to those now in use for measuring temperature, but some changes will have to be made. to adapt it for using alternating current. The new and important feature of this method is the use of two cells containing liquids of nearly the same properties, which make it possible to compensate almost completely for the large temperature coefficient of sea water. The two cells are placed in a uniform temperature bath and the error will be only that due to the small differential temperature coefficient of the two solutions. EXPERIMENTAL WORK Preliminary experiments to test the general feasibility of the method showed: 1. Good balances can be obtained with a simple Wheatstone 3 THURAS, A. L., Journ. Wash. Acad. Sci. 7: 605. 1917. bridge circuit containing the two electrolytic cells, using either a telephone at 500 cycles per second or an alternating current galvanometer at 60 cycles per second as a detector. 2. The temperature compensation is sufficient. For the maximum difference in salinity the lack of compensation did not exceed 0.03 in salinity (0.03 gram of solids per kilogram of water) for a change of 10°C. 3. No appreciable change in balance due to the flow of the sea water through the open cell was obtained. 4. To obtain a continuous record of salinity an alternatingcurrent galvanometer similar to the usual direct-current galvanometer is needed to operate the recorder. This galvanometer was constructed of the electromagnet moving coil type, and had a sensitivity and other operating constants as good as those of the direct-current galvanometers now used. After these preliminary experiments on some temporary cells had shown the feasibility of the method a more careful study was made of certain sources of error in order to obtain data upon which to base the design of the final cells. These effects are: 1. Heating effect of the current in the cells; 2. Temperature lag of the sealed cell when the sea-water temperature in the bath suddenly changes; 3. Time necessary for the resistance ratio to reach its true. value if the sea water passing through the open cell changes in salinity. In the ordinary conductivity measurements performed in a laboratory the heating effect of the current can be made negligible by using a sufficiently sensitive galvanometer or telephone receiver, but with the less sensitive recording galvanometer this current must be much larger and consequently requires a specially designed cell to dissipate the heat developed. From experiments on differently shaped cells the heating coefficients, i.e., temperature rise per watt dissipated in the cells, were found to be approximately inversely proportional to the diameters and lengths of the cells. Since the length of cell is limited by 4 WEIBEL, E. E., Bureau of Standards Sci. Paper No. 297. p. 23. 1917. |