Theoretical Analysis of the AC/DC Transfer Difference of the NPL Multijunction Thermal Convertor over the Frequency Range DC to 100 kHz

Measurements of the rms values of alternating currents are frequently made by means of thermal transfer instruments. At low and midfrequencies Thomson heating can have an important influence on the ac/dc transfer difference of these devices, whereas at higher frequencies the variation with frequency of the effective resistance of their heaters is the most decisive factor. The present paper examines the influence of both Thomson heating and the reactive components of the heater on the performance of the NPL multijunction convertor, and sets out the theoretical analysis that supports the claim that its ac/dc transfer difference does not exceed 1 or 2 ppm up to a frequency of 100 kHz.