B. D. Josephson preceding lecture( 8). Pippard(9) hadconsidered the possibility that a Coo- per pair could tunnel through an insulating barrier such as that which Giaever used, but argued that the probability of two electrons tunnelling si- multaneously would be very small, so that any effects might be unobservable This plausible argument is now known not to be valid. However, in view of it I turned my attention to a different possiblity, that the normal currents through the barrier might be modified by the phase difference. An argument in favour of the existence of such an effect was the fact that matrix elements for processes in a superconductor are modified from those for the corres ing processes in a normal metal by the so-called coherence factors, (3 are in turn dependent on 49(through the uks and uks of the BCs theory At this time there was no theory available to calculate the tunnelling current. part from the heuristic formula of Giaever, (7)which was in agreement with experiment but could not be derived from basic theory. I was able however, to make a qualitative prediction concerning the time dependence of the current. Gor'kov(4) had noted that the F function in his theory should be time-dependent, being proportional to e-2ijge, where u is the chemical potential as before. (10) The phase a should thus obey the relation a中t=-21h while in a two-superconductor system the phase difference obeys the relation (4)=2eh here V is the potential difference between the two superconducting regions so tha Ad=2evt/h+const. Since nothing changes physically if Ag is changed by a multiple of 2, I was led to expect a periodically varying current at a frequency 2e v/h The problem of how to calculate the barrier current was resolved when one day Anderson showed me a preprint he had just received from Chicago, (11)in which Cohen, Falicov and Phillips calculated the current flowing in a super conductor-barrier-normal metal system, confirming Giaever's formula. They introduced a new and very simple way to calculate the barrier current-they simply used conservation of charge to equate it to the time derivative of the mount of charge on one side of the barrier. They evaluated this time deriva- tive by perturbation theory, treating the tunnelling of electrons through the barrier as a perturbation on a syste In consI isting of two isolated subsystems between which tunnelling does not take place I immediately set to work to extend the calculation to a situation in which both sides of the barrier were superconducting. The expression obtained waspreceding lecture (8). Pippard (9) ha considered the possibility that a Coo- d per pair could tunnel through an insulating barrier such as that which Giaever used, but argued that the probability of two electrons tunnelling si￾multaneously would be very small, so that any effects might be unobservable. This plausible argument is now known not to be valid. However, in view of it I turned my attention to a different possiblity, that the normal currents through the barrier might be modified by the phase difference. An argument in favour of the existence of such an effect was the fact that matrix elements for processes in a superconductor are modified from those for the correspond￾ing processes in a normal metal by the so-called coherence factors, (3) which are in turn dependent on A@ (through the U/~‘S and u,,.‘s of the BCS theory). At this time there was no theory available to calculate the tunnelling current. apart from the heuristic formula of Giaever, (7) which was in agreement with experiment but could not be derived from basic theory. I was able. however, to make a qualitative prediction concerning the time dependence of the current. Gor'kov (4) had noted that the F function in his theory should be time-dependent, being proportional to e-2ip”/h, where µ is the chemical potential as before. (10) The phase F should thus obey the relation (3) while in a two-superconductor system the phase difference obeys the relation where V is the potential difference between the two superconducting regions. so that Since nothing changes physically if A@ is changed by a multiple of 2p, I was led to expect a periodically varying current at a frequency 2eV/h . The problem of how to calculate the barrier current was resolved when one day Anderson showed me a preprint he had just received from Chicago, (11) in which Cohen, Falicov and Phillips calculated the current flowing in a super￾conductor-barrier-normal metal system, confirming Giaever’s formula. They introduced a new and very simple way to calculate the barrier current-they simply used conservation of charge to equate it to the time derivative of the amount of charge on one side of the barrier. They evaluated this time deriva￾tive by perturbation theory, treating the tunnelling of electrons through the barrier as a perturbation on a system consisting of two isolated subsystems between which tunnelling does not take place. I immediately set to work to extend the calculation to a situation in which both sides of the barrier were superconducting. The expression obtained was of the form