Thermocouples are connected as shown in Fig. 12.14b. The sensing junction and
the reference junction are connected in series. When the junctions are at the
same temperature the voltage output from the junctions is zero, and the output
from the amplifier is zero. When the junctions are at different temperatures,
there is a differential voltage at the input to the amplifier that is amplified and
converted to a temperature reading. To make this an absolute reading the temperature
of the reference junction is required.