PTB News 3/2000 (554 kB) PTB News 3/2000, English edition, Issue December 2000With the application of a new Josephson voltage standard as a fast reversible voltage source, thermoelectric effects in thermal converters were verified. Thus, the uncertainty of the measurement of alternating voltage and alternating current is reduced to less than 0,1 µV/V and 0,1 µA/A, respectively.
Precise measurement of alternating voltage and alternating current is performed with thermal converters. The Joule heat in a resistor, produced by the ac quantity, is compared with a well-known dc quantity by measuring the increase in temperature of the resistor. Thermoelectric effects, such as the Thomson effect, however give rise to slightly different temperature profiles for direct or alternating current. These effects can be determined by the «Fast Reversed DC Method» (FRDC). The method is based on the fact that the Thomson effect depends on the direction of the current and it modifies the temperature profile with the thermal time constant of the converter. If the direction of current is reversed with increasing frequencies, the modification of the temperature is suppressed. Thus the Thomson effect can be determined by varying the reversing frequency.
So far the reversible voltage source, necessary for the FRDC method, was realised with semiconductor components and the positive and negative amplitudes had to be adjusted exactly. Now, for the first time the FRDC source has been realised with a Josephson array of the layer sequence superconductor-insulator-normal conductor-insulator-superconductor (SINIS). This SINIS source provides equal output voltages for both polarities with the precision of a quantum standard. The polarity can easily be changed by reversing the bias current of the Josephson array. This technique is not possible with conventional Josephson arrays because of their hysteretic characteristic.
Measurements of two different planar multijunction thermal converters with reversing frequencies from 0,5 Hz up to 200 Hz showed thermoelectric effects of less than 0,2 µV/V. Because of the quantised voltage of the Josephson circuit, the standard measurement uncertainty is reduced.
In result the application of the Josephson-FRDC- source reduces the uncertainty of transfer measurements with thermal converters. Further, the good agreement of the new Josephon-FRDC-source with the semiconductor source used so far increases the confidence into the FRDC method.