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Thermocouple Created 2026-10-14 Updated 2026-10-14

A thermocouple uses two different electrical conductors and the Seebeck effect to infer a measuring-junction temperature from a voltage and a known reference temperature. The reference material and the reference temperature are distinct choices: B specifies the comparison Seebeck coefficient, while

T_{c}

specifies the lower limit of the integral. The two-material experiment in Seebeck effect gives the geometry, sign convention, and calibration limitations.

The useful measurement is a pair: the raw terminal electric potential difference and the thermocouple reference junction temperature. For the A-positive terminal order and homogeneous-wire assumptions of Seebeck effect, write the open-circuit electromotive force as

E (T_{h}, T_{c}) = \int_{T_{c}}^{T_{h}} [S_{A} (T) - S_{B} (T)] d T .

Here

T_{h}

is the thermocouple measuring junction temperature and

T_{c}

is the common temperature of the reference connections. The subscript

h

names the measuring end; it need not be hotter. A voltmeter senses the complete connected thermoelectric path through the wires and its leads, in the electrochemical potential convention already described under Seebeck effect. It does not measure an absolute temperature, the individual Seebeck coefficient of A, or an isolated contact electric potential.

Figure 1.
Thermocouple wiring with two copper-lead junctions at a common reference temperature
. Original schematic. The isothermal terminal block contains electrically separate A–Cu and B–Cu transitions and a local temperature sensor. Both homogeneous copper leads end at the same meter-input temperature $T_{m}$ , so their thermoelectric contributions cancel. The displayed $+ 2.4 mV$ is calculated from a stipulated constant difference of Seebeck coefficients, not measured data or a calibration of actual materials. Details.

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Thermocouple measuring junction 2026-10-14

The thermocouple measuring junction joins A and B at the location whose temperature is to be inferred. Its own temperature is what the electrical measurement estimates. To infer a surrounding object's temperature, the junction must approach thermal equilibrium with that object: insufficient immersion, heat conduction along the wires, or a changing environment can keep the junction at another temperature. A stable electrical display alone does not establish that this thermal condition holds. The two-heat bath procedure and terminal-reversal controls under Seebeck effect provide the starting experiment; the practical issue here is the reference and conversion chain.

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