The temperature dependence and magnetic field dependence of the coupling force have been measured in two transformer systems, one with primary and secondary film thicknesses dₚ and dₛ comparable with the superconducting penetration depth λ(dₚλₚ~dₛλₛ~1) and one with film thicknesses small compared with the penetration depth (dₚλₚ~dₛλₛ~10^-1). The results show that (i) the coupling force is nearly two orders of magnitude greater in the thick-film transformer than in the thin-film transformer; (ii) there exists a transition field above which the coupling force decreases very rapidly, typical values for the transition field being quite low (~0.5 G); and (iii) the temperature dependence of the coupling force over much of the range investigated is simply proportional to the temperature dependence of λₚ^-2λₛ^-2. A Gibbs-free-energy theory describing the coupling force has been numerically evaluated and compared with these data. The results show that the theory can be used to predict accurately both the absolute magnitude and the field and temperature dependences of the coupling force for wide-ranging values of the operating parameters.
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Ekin et al. (1975) studied this question.