142 



2. Put into the measuring or X cell sea water of known salinity and. 



preferablj' of such salinity as cori*esponds to a reading in the 

 vicinity of 500 as given by the table or curve, first having rinsed 

 the X cell with sea water of substantially the same salinity. 



3. Set the reading of the slide wire to correspond with the known salinity 



as given by the table or curve. 



4. Balance the bridge according to instructions given above, making the 



adjustments of Q in each case instead of the slide wire which must 

 be kept at the reading corresponding to the known salinity. 

 Should this not be possible, very probably the salinity of the .sea 

 water used in the Y cell is not suitable. 



5. Withdraw from the X cell the solution of known salinity. Rinse with 



some of the sea water whose salinity is to be measured, then fill 

 with the sea water. 



6. Balance the bridge according to directions given above. 



7. Record the reading of the slide wire. 



8. From the curve or table find, by interpolation if necessary, and 



record the salinity of the sea water. 

 In each case, before making the final balance of the bridge, sufficient time 

 should be allowed for the sea water in the two cells to come within 0.01° C. 

 of the same temperature, and the bath should be regulated at a temperature 

 differing from 25° C. by not more than a few hundredths of a degree. Unless 

 the X and Y cells have very nearly the same constants, it may be advisable 

 or even necessary to take their differences into consideration in deciding upon 

 the salinity of the sea water to be used in the Y cell. In any case this cell 

 should have a resistance such that the bridge may be balanced with compara- 

 tively small setting for Q when the slide wire is set at a reading corresponding 

 to the salinity of the sea water in the X cell. The following table for con- 

 verting readings of the apparatus to salinities, is based on values for the 

 conductivity published by Thuras in the Journal of the Washington Academy of 

 Science, and upon constants of the apparatus determined in the Bureau of 

 Standards. The way in which these enter into the construction table is ex- 

 plained in Appendix 1. 



Table for conversions of ai)paratus readings to saliuities 



CARE OF APPAUATUS 



In order to maintain the electrical insulation it is advisable to keep the 

 inside of the cabinet as free as possible from salt, and thoroughly dry, at least 

 whenever in use. Any sea water spilled inside the cabinet should be taken up 

 promptly and tlioroughly. Tiie electrical terminals of the cells should be in- 

 spected occasionally, or wiped out with a small amount of cotton waste. Sea 

 water left in these terminals in contact with the mercury and copper causes, 



