143 



apid corrosion. To facilitate keeping' tlie caliinet dry, no water either iresli 

 ir salt should be kept on the inside unless properly sealed, and in addition the 

 aside should, during the least part of the time, be kept at a temperature above 

 hat prevailing outside the ship. In any case care should be taken to see that 

 he water bath does not freeze. The electrical contact in the thermoregulator 

 hould be inspected occasionally and cleaned if necessary. 



Appendix I 



RELATIONS 13ETWEEN BRIDGE HEADINGS AND SALINITIES 



A simple plan of the bridge is shown in Diagram No. 1, wiiere X is the re- 

 ;istance of the bridge arm. including the cell in which the sea water whose cou- 

 luctivity is to be measured is placed, and Q + Y is the resistance of the arm of 

 he bridge containing the auxiliary or comparison cell. M and A" are the 

 ■esistances of the ratio arms when the slide is set at zero. If, then. Q is ad- 

 usted so that the bridge is balanced, from the Wheatsone bridge equation it 

 ollows that 



tvhere X' represents the resistance of the bridge arm when the cell contains 

 sea water of resistivity R' and a salinity S'. 



If now the sea water is changed to one of greater salinity and the bridge 

 is balanced by changing the position of the slider from a reading of zero to 

 a reading r 



Wliere m is the resistance of one division of the slide wire divided by 

 M, and N is the resistance of one division of the slide wire divided by N, 

 from equations (1) and (2) it follows that— 



X=X'{l—{)n-\-n ) r+ ( mn + ir) r— ( mn'-i-n^) r') (3) 



or as the resistivities R' and R of the tw(. samples have the same relation 

 as their resistances, since the same cell is used in the two cases, 



R=ir (1 — ( m+ii ) r+ ( iini+ir ) r—( m »"+«') r') (4) 



Since it has been found that the relation lietweeu the salinity and resistivity 

 is approximately linear it may be represented over the range in which we 

 are concerned by a second-order equation of the following form : 



A'-Z?'(1+A(N— «^')+B(.S'— ^")') (5) 



Where S is the salinity of the sea water having the resistivity R. and S' is 

 the salinity of the sea water having the resistivity R', from equations (4) 

 and (5) it follows that— 



S = - (- + ") , + (-^^^) .3 _ (--H-Jl^) r-liS- Sy+S^ 



(6) 



or 



8 =ar+h r + er= + <l-< + S' ( 7 ) 



where, 



vi-\-n ,, mn-\-n- ?n?r-+w5 B 



s = S-S', a= ^, ''6 = — ^, c = — -^ » andrf=- -^- 



