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From precision measurements of the quantized Hall resistance of a GaAs heterostructure the following value for the fine-structure constant a has been obtained a-' = 137.035 992 x (1 * 2.7 x

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The PTB has improved the maintenance of the volt with the aid of a new standard cell enclosure containing 3 2 cells monitored by the Josephson reference voltage. The paper gives detailed information on the construction and the operational experience of the thermo-regulated air-bath enclosure. Since February 1976, a group of 25 standard cells in the new enclosure, in conjunction with the Josephson voltage standard, has represented the volt at the PTB. The extremely small drift of the mean value of the emf of this group is considerably smaller than one part in lo8 per year.

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8. -van Dijk, H., Durieux, M.: Differences between the CCT-64 scale of temperatures and the gas thermometer scales of NBS, KPL, PRMI, PSU, in the temperature range below 91 "K, Document No. 1 presented to Comitb Consultatif de Thermombtrie, 8th session, 1966.