  We have measured the linear dielectric susceptibility of two molecular glass
formers close to Tg in order to estimate the size of the dynamically correlated
clusters of molecules which are expected to govern the physics of glass
formation. This size has been shown to be related to the dynamic dielectric
susceptibility dEps(w)/dT (Eps : dielectric susceptibility, T : temperature,
w&#61472;: frequency). To allow for an accurate determination of the T
derivative, we scanned the interval 192 < T < 232 K every 1 K for glycerol and
159 < T < 179 K every 0.5 K for propylene carbonate. The resolution on T
variations was about 1 mK. The result for glycerol is that the number of
correlated molecules increases by a factor 3 when T goes from 226 to 195 K. It
has been shown that the non-linear susceptibility provides a direct measurement
of dynamic correlations. To measure it, we used a standard Lockin technique
yielding the third harmonic of the current flowing out of a capacitor. We
obtained only an upper limit on the ratio of the third to the first harmonic,
due to the non-linear response of standard electronics.
