Module 4 : THERMOELECTRICITY
Lecture 21 : Seebeck Effect
where the coefficient of proportionality is known as the thermoelectric power or the Seebeck coefficient. The term thermoelectric power is a misnomer because it does not measure any power and is measured in volt/ $^\circ$K. By convention, Seebeck coefficient's sign is the sign of the potential of the cold end with respect to the hot end. Thus if is positive, convertional current flows from A to B at the hot junction. Seeback coefficient is not a constant but is dependant on temperature. The temperature dependence of a commercial thermocouple is usually expressed as a polynomial expansion in powers of temperature $T$. For instance, for a thermocouple with Platinum as one of the metals and an alloy of Pt-Rh (90:10) the open circuit voltage is given approximately by a quadratic
 
\begin{displaymath}V = c + aT + bT^2\end{displaymath}
  so that the thermoelectric power is given by
 
\begin{displaymath}\frac{dV}{dT} = a + 2bT\end{displaymath}
  The relationship between $V$ and $T$ is a parabola. The temperature $T_n = -a/2b$ at which the thermoelectric power is maximum is called the neutral temperature . The temperatures $T_i = T_0$ and $T_i= T_0 - a/b$ at which a small change in the difference of the junction temperatures leads to a change in the sign of emf is called the inversion temperature.
 
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