Onset of superconductivity — electrical resistivity outcome When a material transitions into the superconducting state (below its critical temperature), its electrical resistivity becomes:

Introduction / Context:
Superconductivity is characterized by two hallmark properties: zero direct-current resistivity and the Meissner effect (expulsion of magnetic flux) below a critical temperature T_c. The resistivity behavior distinguishes superconductors from merely very good conductors.

Given Data / Assumptions:

• DC measurement regime (not high-frequency where other effects arise).
• Temperature below the material’s critical temperature T_c and below critical field/current limits.
• Ideal superconducting state (no flux flow losses).

Concept / Approach:
In the superconducting state, charge carriers form Cooper pairs that move without scattering, leading to exactly zero DC resistivity. This allows persistent currents to flow indefinitely in closed loops, observed experimentally for months to years with no measurable decay.

Step-by-Step Solution:
Above T_c: material has finite resistivity ρ_n (normal state).Cool below T_c with no excessive magnetic field/current: transition to superconducting phase.Measured DC resistivity drops discontinuously to ρ = 0 within instrument precision.Therefore, the correct outcome is zero resistivity, not just “very small”.

Verification / Alternative check:
Persistent current and levitation demonstrations require truly zero DC resistance; any finite resistance would dissipate energy and decay currents rapidly.

Why Other Options Are Wrong:
“Very small” or percentage fractions imply finite resistance inconsistent with superconductivity; DC zero is the defining property.

Common Pitfalls:
Confusing AC losses or flux-flow resistance in type-II superconductors with DC zero-resistance; ignoring the role of critical field and current density, which can quench superconductivity.

Final Answer:
zero