Abstract: Apparatus for identifying the metal alloy composition of a test piece. Two thermoelectric probes are brought into contact with the test piece. Both of the probes are heated, one of which is heated to a predetermined temperature greater than the temperature of the other probe. The probes comprise electrodes of a thermally conductive material having in thermoelectric contact therewith, respective thermoelectric circuits having differing predetermined thermoelectric characteristics. A variable voltage divider electrically couples the respective thermoelectric circuits of the two probes. The output terminals of the voltage divider networks are connected in series opposition through a null meter or indicator which displays the test result. The voltage divider networks of the respective probes are mechanically or electrically slaved such that they are adjusted in synchronism, thereby desensitizing the apparatus to changes in test temperature, ambient temperature or the temperature of the respective test pieces.