could be coupled with a temperature sensor to give values of density 

 and temperature from which salinity could be derived. 



A simple form of the densitometer would consist of a tube rigidly 

 clamped in a heavy base and made to vibrate at its resonant frequency 

 by feeding the output of a pickup coil into an amplifier which then 

 excites a driving coil. If the tube is made of a nonmagnetic material, 

 a short sleeve of iron or steel is firmly attached to the free, vibrating 

 end. This is in essence an electrically driven tuning fork. Such devices 

 have a very sharp resonance peak and are, therefore, inherently very 

 stable in frequency. The frequency at which the rod vibrates is deter- 

 mined by its length, its mass per unit length, and the strength of the 

 material from which it is constructed. The stability with changes of 

 temperature is determined by the variation of the above quantities with 

 temperature. For the liquid- filled tube the mass per unit length and, 

 therefore, the resonant frequency will vary with the liquid density. 

 Numerous problems would have to be resolved before such a concept 

 can be developed into a working instrument. Richardson states that 

 such an instrument probably would not be applicable to the direct 

 measurement of the absolute density but that it should be adaptable to 

 measuring changes in density. 



C. LABORATORY INSTRUMENTS 



Conductivity instruments also have been developed for the analysis 

 of water samples in the laboratory, either aboard ship or ashore. A 

 number of such instruments are discussed below. 



1. "Wenner-Smith-Soule conductivity bridge (modified) 



The most noteworthy of the conductivity instruments is the 

 modernization of the Wenner-Smith-Soule conductivity bridge which 

 had been used successfully aboard ship by Coast Guard oceanographers 

 since the early 1920's. Improvement of this bridge was initiated by 

 Schleicher and Bradshaw at the Woods Hole Oceanographic Institution 

 in the early 1950 T s. Paquette at the University of Washington made 

 additional improvements. 



Several features make the new instruments quite different from the 

 original Coast Guard model. One of the main differences is the use 

 of the servomechanism to complete the balance of the conductivity 

 bridge automatically. Although it makes the design electronically 

 more complicated, this feature reduces the strain on the operator that 

 is encountered in the manual, audible method of balancing and lessens 

 possibilities of human error. Also, the improved bridges avoid the 



IV- 7 



