  A novel approach to calibrate the sensitivity of a differential thermometer,
consisting of several thermocouples connected in series (thermopile), has been
developed. The goal of this method is to increase the accuracy of small
temperature difference measurements ($\Delta T \leq 1\:\textrm{K}$), without
invoking higher sensor complexity. To this end, a method to determine the
optimal temperature difference employed during the differential measurement of
thermoelectric sensitivities has been developed. This calibration temperature
difference is found at the minimum of combined measurement and linearization
error for a given mean temperature. The developed procedure is demonstrated in
an illustrative example calibration of a nine-junction thermopile. For mean
temperatures between $-10\:^{o}\textrm{C}$ and $15\:^{o}\textrm{C}$, the
thermoelectric sensitivity was measured with an uncertainty of less than $\pm
2\:\%$. Subsequently, temperature differences as low as $10^{-2}\:\textrm{K}$
can be resolved, while the thermometer used for the example calibration was
accurate only to $\pm 0.3\:\textrm{K}$. This and higher degrees of accuracy are
required in certain research applications, for example to detect heat flux
modulations in bifurcating fluidic systems.
