The results of a systematic study of the design considerations of a low-temperature tunnel diode oscillator are presented. The calculated circuit performance satisfactorily describes the measured frequency noise and dependence on bias voltage, magnetic field, and temperature. The overall performance allows measurement of changes in the resonant frequency of an LC circuit with a precision of 0.001 ppm. One can thereby detect extremely small changes in a number of material properties such as thermal expansion, surface impedance, and electric and magnetic susceptibilities.

We report the breakdown of the nearly dissipationless quantum Hall effect into a set of distinct, quantised dissipative states in a wide, high-quality GaAs/AIGaAs sample. We found 35 dissipative states on one plateau and 9 on another plateau which have longitudinal voltage drops accurately quantised in units of ho,/e to within our 0.6% measurement uncertainty. This voltage quantisation implies that the energy dissipation per carrier is quantised in units of the Landau level spacing hw,.

Absfruct-The quantum Hall effect is being used to monitor the U.S. legal representation of the ohm, or as-maintained ohm, RNns. Measurements have been made on a regular hasis since August 1983. Individual transfers between the quantized Hall resistance R,, and the five 1-O resistors which comprise nNns can now he made with a total one standard deviation (lo) uncertainty of k0.014 ppm. This uncertainty is the root-sum-square of 32 individual components. The time

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