McLaren and Murdock [2] noted hysteresis effects in the AufPt thermocouple amounting to 20-50 mK at the Sb and Ag points, and attributed it to differential thermal expansion of Pt and Au. Such effects, observable as a difference in the measured temperature profile in a fixed-point cell, depending on the direction of immersion (insertion or withdrawal), were overcome by using a stress- relieving coil at the thermocouple junction. McLaren and Murdock used coils consisting of about five turns of Pt wire of diameter 1/4 of that of the thermoelements, i.e. about 0.13 ram, a design adopted by others for the Au/Pt and Pt/Pd thermo- couples (see Table 1). In addition, they used a lead harness for the flexible section of thermocouple just beyond the alumina insulator to minimize deformation of wires during assembly and while changing the immersion within a furnace. For greatest benefit, a bar clamp was included. The expansion coefficients of various elements are given in Fig. 4, which includes data for high- density polycrystalline alumina. Clearly, the ther- mal expansion of Au is twice that of Pt, but equally, it is more than twice that of alumina. For measurements with a thermocouple mounted verti- cally, as it would be in a standard fixed-point cell, the relative expansion of the alumina will have little effect, provided the bore is large enough, perhaps > 1 mm, and a stress-relieving coil is used. However, the effect of any expansion relative to that of the alumina should be considered for thermocouples mounted horizontally. Even though the expansion coefficient for silica is much less than that of the elements of Fig. 4, its inner surface would be smoother than that of polycrystalline alumina and this may be sufficient to overcome any tendency to cause compressive stress in the thermoelements. Notice from the figure, that the expansion of Pd relative to Pt is only half that for Au/Pt, and a coil may not be required for the Pd/Pt thermocou- pie, although the expansion in alumina may still be a problem when horizontal. Notice, also, that the differential thermal expansion would be less of a problem if a pair of thermoelements consisted of Pt, Rh and/or Ir and if they were mounted in alumina.