SECT. 1] 



THE FLOW OF HEAT THROUGH THE FLOOR OF THE OCEAN 



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to imperfect levelling and must withstand the shock it experiences when the 

 probe runs into the sediment. The zero position on the record is not known, but 

 the deflection may be determined by reversing the thermocouple connections 

 to the galvanometer. Electronic amplification before recording is not practicable 

 for such small e.m.f.'s. The advantages of the thermocouple are that it is itself 

 simple and robust, and that it measures a temperature difference directly and 

 does not require it to be found as the difference of two temperatures measured 

 separately. 



Fig. 1. Probe for measuring temperatixre gradient. (By courtesy of British Petroleiim Co.) 



A thermistor measures a single temperature and therefore needs careful 

 calibration or matching if it is one of a pair used to determine a small tempera- 

 ture difference. The resistance change of a thermistor is typically 5%/°C. The 

 e.m.f. that can be obtained from a bridge of which it forms one arm is hmited 

 by the heating produced by the measuring current, but is of the order of 3 mV 

 for 0.2°C. This e.m.f. may be used directly to drive an insensitive and robust 

 galvanometer of the type used in seismic prospecting or the bridge may be 

 made self-balancing by an amplifier connected in a servo-loop ; a pen recorder 

 may then be used. If an amplifier is to be used the bridge may be fed by a.c, 



