Measurements of the attenuation of a magnetic field in superconducting tin have revealed the sign reversal predicted by nonlocal theory. 1 These experiments give a straightforward demonstration of a nonlocal Meissner effect in tin. While this demonstration comes too late to be of primary importance, it suggests that the form of the penetration curve can be investigated in further experiments, leading to more detailed information on the integral kernel in the theoretical expression relating the density of supercurrents and the magnetic vector potential.In this experiment, tin films were evaporated onto the outside surface of a rotating glass substrate to obtain long hollow cylindrical films. They were then cut from the vacuum system and cooled down to liquid helium temperatures. A magnetic field H a at frequency y = l.lxl0 5 cycles/ sec was applied parallel to the axis of the cylinder; the magnetic field H^ which penetrated through the films into the interior of the hollow body was detected. (Although the sign reversal is in fact a dc effect, it should be frequency-independent below microwave frequencies. The choice of v was dictated by instrumental considerations.)The equipment 2 was designed according to the results of our numerical calculations. 2 ' 3 A detailed account of the experimental work will be published later; it is, therefore, only briefly described here.The current for the exciting field was produced by a sine wave generator; a maximum field strength of # a = 30 Oe was available. The (strongly attenuated) magnetic field which penetrates the superconducting film was detected by a pickup coil placed inside the superconducting cylinder. This signal, tuned with an external capacity and amplified in a tuned amplifier, was compared with the drive current in the exciting coil on a dual beam cathode-ray scope with external triggering. Both the amplitudes and the phases of the two voltages could then be compared. Figure 1 shows a plot of measured reciprocal field attenuation ratios E a /U^ versus temperature T for two films. In the case of film I (31 000 A thick), the field attenuation ratios are detectable only down to 3.4°K. At lower temperatures, H^ becomes immeasurably small; the plateau below 3.4°K, corresponding to H a /Hj = 1 x 10 9 , reflects the limitation of our equipment. Film II had a thickness of 18 700 A and a residual resistance ratio of r = 0.03. Measured reciprocal field attenuation ratios for this film are appreciably smaller than lxlO 9 at all temperatures measured, and thereby well within the demonstrated sensitivity of the equipment. Figure 2 shows oscilloscope photographs of the exciting current and the corresponding pickup voltage taken on film II at T = 3.41°K and T = 2.88°K, respectively. Taking the current signals as reference, the 180° shift in the corresponding voltage signals is at once evident.An analysis of the phase shift between exciting current and pickup voltage involves knowledge of the absolute phase shift of the apparatus. It turns out that the external and attenuated mag...

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