52 



In anticipation of the arrival of the new bridge 14 samples of sea 

 water of well distributed salinities had been saved during the AprU 

 oceanographic cruise. 



Each of these samples was measured in the bridge, measured twice 

 by titration and then measured a second time in the bridge. These 

 measurements gave 14 points on the calibration curve of the bridge in 

 addition to an arbitrarily selected point of 35.00 %o salinity equal 

 to a dial reading of 50.15. 



In this instrument two adjacent arms of a Wheats tone bridge are 

 made up of two similar electrolytic cells, one of which contains a 

 sample with whose salinity we are concerned and the other contains 

 sea water the salinity of which is of no great importance. The other 

 two arms are composed of wire-wound impedances, a part of the 

 impedance in each of the last two arms being adjustable by means of 

 dial switches. Let the first cell contain a solution of known salinity 

 S' and conductivity C, and let X' be the impedance of the arm N. 

 Let M' be the impedance of the dial-controlled arm adjacent to the 

 arm containing the first cell, and let Q be the impedance in the fourth 

 arm. Let M' be of such a value that its dial reading corresponds to a 

 salinity of S', and let the bridge be balanced by adjusting the im- 

 pedance Q. Then we can write 



X' = M' (N/Q) (1) 



If now the sea water in the first cell is replaced by a sample of some 

 other salinity, S, and conductivity C, and the bridge is balanced by 

 changing M' to M then 



X=M (N/Q) (2) 



And combining equations (1) and (2) we have 



X=X' (M/M') or C = C' (M'/M) (3) 



The relation between the electrical conductivity and salinity of sea 

 water over the range 30 to 40 %o salinity can be closely approxi- 

 mated by the equation 



C-C (A+BS) (4) 



where A and B are constants and S is the salinity corresponding to 

 conductivity C. From equations (3) and (4) we have 



A+BS=(M7M) or S=(1-A)/B-(M-M')/MB (5) 



But from equation (4) 



S' = (l-A)/B (6) 



