16 



U. S. COAST AND GEODETIC SURVEY 



urements by Ejiudsen by a conductivity method similar to that of 

 Monsieur and Madame Chauchard. They state that Knudsen 

 obtained good results in shallow water. 



A device patented by Alexander McNab is described in Marine 

 Engineering and Shipping Age, for May, 1927, which operates on 

 the electrical conductivity principle and gives a continuous indica- 

 tion of the salinity of the boiler feed water on board ship. 



Regarding the relative conductivities of water samples of various 

 salinities. Monsieur and Madame Chauchard state that they found 

 a change of resistance from 6.3 ohms for sea water to 450 ohms for 

 fresh water from the laboratory tap. In the summer of 1927 the 

 writer measured the resistance at 24.° 6 C. of the four samples of 

 sea water used in the refractometer calibration described earlier in 

 this jDublication. Measurements were made on a Leeds-Northrup 

 portable test set with an alternating-current source and with phones 

 as balance indicator. As electrolytic cell, the tube shown in Figure 

 7 was used. The results of the measurements are shown in Table 5. 



Table 5. — Variation icith salinity at 24-° 6 C. of electrical resistance of column 

 of sea tcater contained in the apparatus shown in Figure 7 



These measurements are described merely to indicate roughly how 

 the conductivity varies with the salinity. However, the indications 

 were that even with this crude arrangement salinity could be meas- 

 ured with an uncertainty not greater than 3 in the third significant 

 figure. 



Any conductivity apparatus must be calibrated by means of meas- 

 urements made with it on samples standardized by other methods. 



METHOD OF BALANCING COLUMNS OF LIQUIDS 



To test the possibility that the salinity of sea water could be 

 measured on board ship by balancing a column of sea-water sample 

 against a column of some standard liquid, the apparatus shown in 

 Figure 8 was purchased by the Coast and Geodetic Survey and was 

 calibrated at 25° C. in the summer of 1927. Kerosene colored with 

 oil red was used in one arm and the sea-water sample in the other; 

 the two liquids did not come into contact. The method was found to 

 be much more convenient than was expected, a complete set of opera- 



