734 



[CHAP. 22 



C. Isotherm Follower 



A more direct instrument for measuring the temperature of internal waves is 

 the isotherm follower (LaFond, 1961), which automatically traces the vertical 

 oscillations of an isotherm with reference to time. The isotherm follower com- 

 prises four parts : (1) a sea-sensing unit ; (2) an electric winch containing a cable 

 to which the sea-sensing unit is attached; (3) electronic components (servo- 

 mechanism, amplifiers, power supply, controls, etc.); and (4) two recorders 

 (depth and temperature) (Fig. 3). 



Fig. 3. Isotherm follower assembly. 



A. Sea-sensing unit C. Electronic unit 



B. Electric winch D. Depth and temperature recorders 



The sea-sensing unit contains a thermistor bead balanced in a bridge circuit 

 with a resistance corresponding to the desired isotherm temperature. If the 

 bridge becomes unbalanced, a thyratron tube is fired. This activates a winch 

 and causes it either to wind in or let out the sea unit. Thus the sea-sensing 

 unit is "locked" onto the desired isotherm. The depth of the isotherm is con- 

 tinuously recorded on deck by means of a pressure sensor in the sea unit. The 

 net result is a trace of the given isotherm depth, eflfective to 600 ft, with 

 reference to time. 



