= Thermocouple
{wiki}
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
$$
E(T_h,T_c)=\int_{T_c}^{T_h}[S_A(T)-S_B(T)]\,dT.
$$
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>.
\Image[/thermocouple-reference-junction-wiring.png]
{title=<Thermocouple> wiring with two <copper>-lead junctions at a common reference <temperature>}
{description=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 $+2.4\,\mathrm{mV}$ is calculated from a stipulated constant difference of <Seebeck coefficients>, not measured data or a calibration of actual materials.}
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