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A new coaxial measurement system has been developed to investigate the ac longitudinal resistance along the high-potential side of the quantum Hall resistance (QHR). A novel equivalent circuit of the QHR is used to analyse the ac measurements of the longitudinal resistances along both the low-and high-potential sides of the QHR sample. In addition, a bridge for the measurement of ac contact resistances of the sample is presented. For the first time, it is now possible to perform all the ac measurements whose dc equivalents are well established for reliable dc quantum Hall measurements. While the ac longitudinal resistances on the high-and low-potential sides of the sample are very similar, interesting differences have been observed at high resolution.

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The ac quantum Hall resistance of several GaAs-based samples has been investigated. The influence of temperature, current and frequency was measured for the plateau . Some measurements were performed for the plateau in order to determine the scaling behaviour with respect to . The plateau edge is shown to depend on the electronic temperature and the current at the plateau edge is distributed over the whole width of the sample. At the plateau centre, edge-current effects become apparent which depend on how close the filling factor is to an integer value. The losses of these edge currents cause the ac quantum Hall resistance to differ from the dc quantum Hall resistance.

The ac quantum Hall resistance measured with frequencies in the range of a few kilohertz shows characteristic deviations from the dc quantum Hall resistance. These deviations indicate ac losses which increase linearly with frequency. To investigate these losses, a Hall bar with an inner and an outer "window" has been tested. When external voltages are supplied to the windows, the ac losses can be adjusted to zero. This is explained by a phenomenological model for the current distribution in the Hall bar and leads to a model for the ac losses.