Below its critical temperature a superconductor carries current with no resistance at all — but only up to a field, and therefore only up to a current. This lab draws the critical surface of a type-I superconductor: the empirical fit Bc(T) = Bc0[1 – (T/Tc)2], good to a few per cent for type-I elements, together with Silsbee's rule Ic = 2πa·Bc(T)/μ0, which is derived in two lines from Ampère's law and is the one exact step in the chain. The screening panel uses the two-fluid fit λ(T) = λ0/sqrt(1 – (T/Tc)4) with λ0 = 100 nm, the sourced order of magnitude for most superconductors — not lead's value, and not any one material's. Type-II materials are quoted with an upper critical field Bc2, a different quantity: neither formula here applies to them.
Where this wire sitsSUPERCONDUCTING: at 4.20 K this 80.00 mT metal holds 52.70 mT, and the surface sees 0.00 mT.
Every figure here is computed from the exact value and rounded once. Re-deriving Ic from the printed Bc does not reproduce it: at 4.20 K the printed 52.70 mT gives 131.7 A where the exact chain gives 131.8 A. The state is decided on the exact comparison too, never on the printed figures — just over Ic the surface field and Bc print the same 2 d.p. string, and at exactly Bc the metal is still superconducting. The margin is a percentage and the field-scale figure is a ratio; they do not divide into one another. That ratio compares Silsbee's Jc with the bare field scale Bc/(μ0λ) and carries no fitted coefficient.