  The state of a sample during crystal growth from high temperature solutions
is not accessible in conventional furnace systems. An optimization of the
growth parameters often requires arduous trial and error procedures in
particular in case of novel multicomponent systems with unknown phase diagrams.
Here we present a measurement technique based on lock-in amplification that
allows for in-situ detection of the liquidus and solidus temperatures as well
as structural phase transitions. A thin, metallic measurement wire is mounted
in close vicinity to the melt. Characteristic anomalies in the time-dependent
electrical resistivity of this wire allow for the detection of latent heat
release without using a reference crucible. The method is implemented in a
'feedback furnace' and enables an adjustment of the temperature profile based
on the occurrence or absence of phase transitions. The absolute temperature
serves as an additional source of information. Obtained phase transition
temperatures are in good agreement with differential thermal analysis (DTA).
