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The Peltier effect can be exploited for cooling. The
sample to be cooled (at temperature ) and a large heat bath
(at temperature ) are connected by an electric circuit built
of two conductors and (Fig. 5).The Peltier heat
is absorbed from the sample at one junction, and transferred to
the heat bath at the other. Let the wires have Peltier
coefficients and . Let's assume for simplicity
that they have equal length and cross section as well as equal
electrical () and thermal conductivity (). The
temperature difference achieved between heat bath and sample also
depends on the magnitude of the electric current. For optimal
choice of this current the maximum temperature difference is given
by

(4) 
According to this formula the cooling effect is strongest if, for
given difference of Peltier coefficients, the electrical
conductivity () is as high as possible and the thermal
conductivity () as low as possible  two opposing
requirements.