A high-temperature superconductor (HTS) is a material that carries current without resistance at temperatures reachable by practical cryogenics rather than only near absolute zero. The rare-earth-barium-copper-oxide (REBCO) coated conductors of the early era operated below about 90 K and carried engineering current densities of a few hundred amperes per square millimetre in strong fields. The conductors assumed by mature autopoietic-industry production push this to of order 1000 A·mm⁻² at 40–60 K in multi-tesla fields, which is the design point on which the splinky guideway is sized. HTS windings can be operated as closed persistent loops, carrying a fixed current indefinitely without a power supply.