urging the probe tips onto the surface of the sample with enough pressure to cause the probes to retract against the resistance of the springs 13 activation of the switch 18 indicates that sufficient pressure has been applied between the probe tips and the unknown specimen to ensure the desired electrical and thermal contact for a reliable measurement to be made the activation of the switch is made to render operable the comparison circuit used to determine the ratio of the emf present between the probes to a voltage derived from the thermocouples 14 fig3 shows a circuit which can be used to determine potentiometrically the voltage ratio from which the unknown specimen can be identified the thermocouples inserted in the probes of fig1 are represented as thermocouple i and thermocouple ii in fig3 the output from each thermocouple is fed across a respective one of a pair of potentiometers 31 and 32 connected in series and referred to herein as trimpots each of these trimpots supplies a voltage to a respective end of a potentiometer 33 the emf generated by the unknown specimen in contact with the probes is balanced against the voltage impressed across the potentiometer 33 in the following manner one of the probes is connected via line 35 to the common.