10 
was desired to have a temperature range of 9 - 19°C, the bridge 
was first zeroed to give zero voltage output at 9°C and then the 
span control was adjusted to give approximately a 20 millivolt out- 
put at 19°C. The adjustment of the zero and full scale values are 
made by inserting, in place of the thermistor, a wire-wound re- 
sistor having resistance first equal to that of the mean thermistor 
values at 9°C, and then equal to that of the mean thermistor values 
at 19°C. A means of field-checking the operation of the bridge is 
provided by inserting wire-wound resistors with resistance cor- 
responding to that of the thermistor at intervals of 1°C. 
Mercury batteries were chosen as power supplies because 
of their voltage stability as a function of time after the initial 
aging. Mallory type RM-42R mercury batteries were used in all 
of the temperature bridges. Only one basic bridge has been dis- 
cussed so far. Where the temperatures of several sensors were 
to be recorded on one recorder system by sequential sampling, 
several bridges were used in parallel, with one common power 
supply. Thus, the positive output for each additional bridge (con- 
sisting of one sensor unit and one linearizing resistor, 7) added 
in parallel, can be referred to one negative output terminal, com- 
mon to all bridges. The values of & , and? , are made small so 
as to provide essentially a zero impedance excitation potential, 
thus reducing any interaction between bridges. 
PRESSURE INDICATING SYSTEM 
It was necessary to record depth variations as a function of 
time along with the temperature records, so that the depth varia- 
tions could be suitably compensated at a later time (see section 
entitled Data Reduction). Since the depth desired was that of the 
temperature sensors, the ideal location of the pressure trans- 
ducer inside the hull would be directly under the strut or tower 
containing the temperature sensors. In this location the errors in 
measuring depth of the sensors, with the submarine not at zero 
bubble, * are reduced to a minimum. By mounting the pressure 
*Submarine at zero bubble means that the longitudinal axis is 
horizontal. 
