The theory of Peltier- and Thomson-effect errors in thermal a.c.-d.c. transfer devices

The reversal differences and non-frequency-dependent transfer errors appearing in thermal a.c.-d.c. transfer devices are known to be due to Peltier and Thomson heating. General expressions have been derived for the transfer errors due to these effects in vacuo-thermo-junctions, taking into consideration the effect of the thermal conductance due to the mid-point thermocouple. The analysis has been extended to the indirectly heated thermistor. Experimental evidence has been provided supporting the theory, and it has been shown that thermal devices having heaters of the binary alloy 80Ni 20Cr are probably free from errors due to these causes. The substantial reversal differences encountered in vacuo-thermo-junctions have been shown to be due to both Peltier and Thomson heating, combined with displacement of the temperature-sensing thermocouple from the mid-point of the heater. There is, however, no direct relationship between reversal difference and transfer error; one can exist without the other.