The technique of derivative thermal analysis, dTA, which plots sample temperature vs rate of change of sample temperature, has been refined in this apparatus to produce calibrated derivative thermal analysis curves in units of degrees and degrees/second. The scaling factors of 1 K=2 mV and 1 K/sec = −100 mV permit calibration to 5273 K, materials permitting, with digital temperature readout. The derivative function provides high sensitivity for detecting very weak fast phase transitions by observing thermal arrests in linear heating and cooling curves. A companion automatic temperature controller holds the furnace temperature constant, or programs it up or down at a linear rate of 0.1 K/sec to 10 K/sec. Light from the incandescent sample and surrounding furnace is divided into two paths by a beam splitter. One path to an optical pyrometer provides temperature calibration. Light in the second path is sensed by photo transistors that measure the sample and furnace temperatures. The temperature signal is linearized by taking the logarithm of the photo transistor current. Operational amplifier circuits establish the desired proportionality to temperature. The sample and furnace temperature signals are differentiated to produce rate signals for dTA display and temperature control. Illustrative dTA curves of the U&sngbnd;C, Nb&sngbnd;C, and Nb&sngbnd;Al systems are included.