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The Josephson effect allows supercurrents to flow through thin tunnel barriers separating two superconductors. The dc critical current depends directly on the quantum phase difference of the superconducting wave function on the two sides of the barrier. This paper is an experimental study of interference phenomena in multiply connected superconductors utilizing Josephson tunneling. The results are a striking confirmation of the extensive phase coherence of the superconducting state. London's theory is used in conjunction with Josephson's prediction to explain the observations. Modulation of the maximum supercurrent flow through junction-pair devices was measured in an applied magnetic field and showed periodicities resulting from quantum wave interference. These interference "fringes" were found to occur even when the magnetic flux is confined to a region not accessible to the superconductor. Another experiment allows the measurement of an interference directly attributable to phase modulation by the superconducting electrondrift velocity. These results offer a direct verification of long-range quantum behavior in the superconducting state and further demonstrate the validity and usefulness of Josephson tunneling methods. JAKLEVIC, LAMBE, MERCEREAU, AND SILVER

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