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Measurements have been made of prebreakdown currents between thoroughly outgassed plane-parallel copper electrodes in ultrahigh vacuum, for electrode separations in the range 0.03 cm to 0.2 cm. These currents are found to be in good agreement with the Fowler-Nordheim theory of field emission from the pure metal surface. From combined measurements of prebreakdown current and breakdown voltage at different electrode separations it is deduced that the cathode microscopic field at breakdown is constant and of magnitude (6±1)×107 V/cm. Spectroscopic measurements of both resonance line absorption and line fluorescence have revealed that the density of neutral copper vapor present in the interelectrode space during the application of electric fields only fractionally (≪1%) less than the breakdown field is considerably less than that necessary for volume ionization to occur. For the same experimental conditions, infrared radiation observations have revealed no hot spots on the anode surface. From the measurements it is concluded that mechanisms involving the amplification of field-emission current in electrode vapor produced by steady evaporation from either the anode or the cathode do not explain the electrical breakdown of vacuum between extended copper surfaces. Further, the production of electrode vapor by the detachment of heated anode material under the influence of the force of the applied electric field is unlikely.