STUDY OF FREE-RANGING SHARKS 



443 



n 



WATER FLOW 



END VIEW 



SIDE VIEW 



Figure 14 Wand-type swimming-speed sensor, magnetically coupled. Motion 

 of the external magnet is duplicated by the internal magnet. System works 

 with very low friction. 



a. Wand (senses water flow) 



b. External magnet (in water chamber) 



c. Internal magnet (in oil chamber) 



d. Compass jewel bearing 



e. Opaque partition (plastic) 



f. Vane (either opaque or of a transparency gradient) 



g. Light source (LED, betalight) 

 h. Photoresistive cell (CdS) 



i. Spring. 



END VIEW 



TOP VIEW 



Figure 15 Compass sensor for telemetry of swimming direc- 

 tion. Comparison of the two photocell readings (two trans- 

 mitter channels) allows unambiguous determination of azi- 

 muth. Employs a circular transparency gradient with no 

 abrupt changes (sectors of maximum and minimum density are 

 180° apart). 



a. Compass jewel bearing 



b. Light sources (LEDs, betalights) 



c. Transparency gradient 



d. Photoresistive cells 



e. Compass magnet 



f. Oil-filled chamber. 



