One device, a MIDWATER NET RELEASE SYSTEM, uses two pres- 

 sure pistons (fig. 18.8). The first ( Q ) compresses and cuts the 

 cod end release which allows the net to filter the watta' inder tcnv 

 (2); the second piston ( 5 ) compresses at a greater depth and sev- 

 ers the frontal towing bridle and the net is pursed off midway (3). 

 The selection of depths is accomplished by inserting rods of var- 

 ious diameters in a secondary hole in the cylinder to arrest the 

 piston. 



In looking for devices to aid in our understanding of the dis- 

 tribution of plankton in midwaters let us consider the use of 

 well-logging or conducting cable. Such a cable would provide a 

 means of nnonitoring depth. This may be easily done by using 

 a PRESSURE POTENTIOMETER (fig. 18.9). Three conductors 

 are used for the operation of the potentiometer, the outer cable 

 armor being the third wire and the ground. The signal from the 

 potentiometer is taken from the wire via the winch-drum axle 

 and a slipring brush contact at the end of the axle. Batteries 

 are maintained topside and depth can be conveniently read in 

 microamperes from a meter. Since the wire has little elec- 

 trical resistance, pressure is a linear function of current. 



18.9 



JOY PLUG 



FIGURE 



PRESSURE 



POTENTIOMETER 



(— > "0 RINGS 



METER 



PRESSURE CASE 



BOURNS PRESSURE 

 POTENTIOMETER 



"0" RING 



© 



PRESS. 



WIRING DIAGRAM 



DEPTH 

 DETECTOR 



=5- HOSE CLAMP 



4-COND. CABLE 



BANJO 



WEIGHT 



