Fig. 1. Salinity transducer probe. 



It will be shown that for a discussion of the 

 bridge null adjustment the transformers may be 

 considered to be ideal. Core losses and magne- 

 tizing reactance in the exciting and pickup 

 transformers shunt the power supply and the null 

 signal, and as such tend to increase the required 

 input power and reduce the circuit gain but do 

 not change the null setting. In a like manner 

 the copper losses (resistance) in the exciting 

 and pickup windings, Ki and n Q may produce small 



' vyvc^- 



NULl ADJUST 



Fig. 2. 



Operating principle of induction 

 conductivity cell. 



changes in the required power and gain but not 

 the null setting. 



The important consideration for the accuracy 

 of the transformer bridge is the stability of the 

 voltage ratio, e u /e s , of the exciting, transformer 

 windings and the stability of the current ratio, 

 i u /i s , at null (zero magnetic flux in the pickup 

 transformer). The factors effecting these ratios 

 are the turns ratios, N u /N s and n u /n s , and the 

 leakage flux, i.e., magnetic flux not common to 

 the pair of windings on each transformer. The 

 turn ratios are determined by the physical con- 

 struction of the induction conductivity cell and, 

 therefore, are constant. By using high quality 

 tape wound cores with high permeability, the 

 leakage flux is reduced to an insignificant por- 

 tion of the total flux. Copper losses are negli- 

 gible in the present design since they each con- 

 sist of only 18 turns of fairly large diameter 



The relationship between the resistance of the 

 sea water path, Ru, and the known variable resis- 

 tance is easily calculated. At null the magneto- 

 motive force (ampere-turns) produced in the pickup 

 transformer core by the sea water path must 

 exactly balance the magnetomotive force produced 

 by winding n : 



Using Ohm's Law: 



*u 



u _ e s 



- *u - p- »B 



or 



fu^j 

 e s n s 



*u = ^V 



(1) 



(2) 



(3) 



12 



