= Meissner effect discovery
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The 1933 report https://link.springer.com/article/10.1007/BF01504252[Ein neuer Effekt bei Eintritt der Supraleitfähigkeit by W. Meissner and R. Ochsenfeld] describes cooling tin and <lead> cylinders in a fixed transverse <magnetic field>. Two parallel cylinders were about $140\,\mathrm{mm}$ long and $3\,\mathrm{mm}$ thick, separated by $1.5\,\mathrm{mm}$. A roughly $10\,\mathrm{mm}$ rotating search coil between them drove a ballistic <galvanometer>. At about $5\,\mathrm{G}$, the reported below/above-transition induction ratios were $1.70$ for tin single crystals and $1.77$ for polycrystalline <lead>. The increased gap <magnetic field> indicates redistribution outside the cylinders, rather than an unchanged frozen field. Coil-winding geometry and noncircular tin specimens limited comparison with the ideal prediction. <Ein neuer Effekt bei Eintritt der Supraleitfähigkeit by Walther Meissner and others>[1933 report, first arrangement; English translation PDF pp. 1–2, printed p. 119, alongside the German text]{ref}
Their second arrangement used a <lead> tube, about $130\,\mathrm{mm}$ long with $3\,\mathrm{mm}$ outer and $2\,\mathrm{mm}$ inner diameters. A rotating search coil could sit inside or alongside it. The bore retained <magnetic flux> after removing the applied <magnetic field>; the coil nearly filled the bore, preventing a test of interior field uniformity. A hole containing <magnetic field> is distinct from <magnetic field> penetrating the surrounding material exhibiting <superconductivity>. <Ein neuer Effekt bei Eintritt der Supraleitfähigkeit by Walther Meissner and others>[1933 report, second arrangement; translation PDF p. 2, printed p. 119, bore retention and explicitly stated spatial-resolution limitation]{ref}
The modern <Field-cooled Meissner experiment> provides a complementary readout. MIT's probe I places a sample in a brass holder, with an $810$-turn test coil of $7.1\,\mathrm{mm}$ inner diameter, $10.5\,\mathrm{mm}$ outer diameter and $12.3\,\mathrm{mm}$ length, inside a longer <solenoid>. A nearby silicon-diode <thermometer> tracks cooling by helium gas. With <lead> still normal, the guide applies about $200\,\mathrm{mA}$ to the <solenoid> and connects the test coil to a current-integrator box and <oscilloscope>. Cooling produces a transient; warming gives the opposite sign. Switching off the <solenoid> while the sample is normal checks the sign. <Superconductivity: The Meissner Effect, Persistent Currents and the Josephson Effects by MIT Department of Physics>[Probe I, §4.2 and Fig. 6, printed pp. 5–6; §4.3 temperature control; §5.2.2, printed p. 10, DC field cooling and sign control]{ref}
The inferred change is in linked <magnetic flux>, not a direct scan of $B(z)$. For a stationary test coil the pulse obeys $\int V\,dt=-\Delta\Psi$, where $\Psi$ is its <magnetic flux linkage>. An integrator or AC-coupled <oscilloscope> needs its transfer response accounted for before extracting this pulse area. Keep the source <electric current> fixed during cooling, monitor <temperature>, and compare warming and cooling slowly enough to reduce sample–<thermometer> lag. An AC <inductance> change alone can reflect changed normal-state <electrical conductivity>; the fixed-field expulsion test distinguishes that alternative. <Superconductivity: The Meissner Effect, Persistent Currents and the Josephson Effects by MIT Department of Physics>[§5.1, printed pp. 8–9, thermometer lag; §5.2.2, printed p. 10, limitation of AC mutual-inductance evidence]{ref} Neither this coil readout nor the historical gap-field measurement resolves the microscopic <London penetration depth> by itself.
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