  We study the influence of the Joule effect on the non-linear behavior of the
transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3
by using the crystalline unit cell parameters as an internal thermometer in
X-ray and neutron diffraction. We develop a simple analytical model to estimate
the temperature profile in the samples. Under the actual experimental
conditions we show that the internal temperature gradient or the difference
between the temperature of the sample and that of the thermal bath are at the
origin of the non-linearity observed in the I-V curves. Consequences on other
compounds with colossal magnetoresistance are also discussed.
