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“thermostat” which in our case can be considered to be the copper electrodes (thickness 6 mm, diameter 20 mm). The resulting average sample temperature increase δT ( t ) is 

<img src='content_image/364662.jpg'>

where the mean dc temperature increase δT$_{0}$ is larger or equal than the ac one δT$_{2}$ , thus at any time δT ( t ) ≥ 0. φ$_{2}$ is a phase shift related to heat transport. As our measurements give the nonlinear dielectric response averaged over the sample volume, δT ( t ) in equation (12) is the temperature increase averaged over the same volume. Using equations (11)-(12), we thus obtain for the nonlinear part of the susceptibility due to heating effects: 

<img src='content_image/364661.jpg'>

This is an upper limit on the contribution of heating to the nonlinear response, since we have assumed that δT ( t ) induces instantaneously a modification of the susceptibility. As already advocated in Ref. [3], this is questionable, specially in what concerns the contribution of δT$_{2}$ ( t ) which should be averaged out because of the finite relaxation time τ of the system (see after eq. 17). By using equations (12-13), one gets : 

<img src='content_image/364660.jpg'>

The four terms in the right hand side of equation (14) give the nonlinear response of the system due to heating that we define in general by 

<img src='content_image/364659.jpg'>

In the right hand side of equation (15) each of the four terms are proportional to E $^{2}$. χ ′ 3 ,h and χ ′′ 3 ,h do not depend on E , while ( δχ ′ $_{1}$)$_{h}$ and ( δχ ′′ $_{1}$)$_{h}$ are proportional to E $^{2}$. Our notations 

