. Electronic apparatus for biological research . Figure Basic null-point and bridge circuits for thermocouple measurements If, however, it is necessary to work at higher sensitivity, then use can be made of the law of intermediate temperatures, which states that if a thermo- couple develops a given voltage between temperatures T^ and Tg, this will equal the sum of the voltages developed between temperatures T^ and T^,, and T^, and T^, where T^, is any intermediate temperature. It is thus possible to calibrate a couple between 0°C and a suitable temperature above it, then to place the cold
. Electronic apparatus for biological research . Figure Basic null-point and bridge circuits for thermocouple measurements If, however, it is necessary to work at higher sensitivity, then use can be made of the law of intermediate temperatures, which states that if a thermo- couple develops a given voltage between temperatures T^ and Tg, this will equal the sum of the voltages developed between temperatures T^ and T^,, and T^, and T^, where T^, is any intermediate temperature. It is thus possible to calibrate a couple between 0°C and a suitable temperature above it, then to place the cold junction, together with a second reference thermometer, into high temperature water and continue the calibration curve in the form of additional voltages for known temperature differences between the mercury thermometers. Alternatively, one can use a potentiometer system to 'back-off' the excess voltage due to the temperature difference {Figure ). The couple can again be calibrated in steps, and for very sensitive work it is advisable to use a high resistance potentiometer to keep down the battery drain, and to include a standard Weston cell, in a separate circuit, with which to check the voltage of the ordinary cell (a standard cell will give a voltage of very 388
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