A superconductor can carry direct current without electrical resistance.

Ordinary conductors resist electrical current and dissipate energy as heat. In the superconducting state, DC electrical resistance vanishes. This is a property of a particular material state, not a claim that every part of a superconducting device operates without losses or energy consumption.

Superconductivity survives only within material-dependent limits. Warming above the critical temperature, applying too strong a magnetic field, or exceeding the allowable current can destroy the superconducting state. A useful device must control all of these conditions rather than merely cool its wires once.

The Meissner effect is another defining feature: a superconductor expels magnetic flux from its bulk when entering the appropriate superconducting state. This magnetic response distinguishes superconductivity from the simpler idea of a hypothetical perfect conductor that merely has no electrical resistance.

Heike Kamerlingh Onnes discovered superconductivity in mercury in 1911 during experiments at very low temperatures. The abrupt disappearance of resistance revealed behavior beyond the gradual resistance changes familiar in ordinary metals. Mercury became the first example of a much wider family of superconducting materials.

Superconducting coils can sustain the large currents needed to produce powerful magnetic fields. Applications include MRI scanners and particle accelerators. These systems exploit the electrical properties of the superconducting state, while also requiring substantial engineering for cooling, mechanical forces, protection, and magnetic-field control.

The phrase zero resistance therefore needs a clear boundary. It describes DC transport in the superconducting material under suitable conditions. Refrigeration equipment, electrical connections, changing fields, and other components can still involve energy costs. A superconducting magnet is not the same thing as an entire machine that consumes no power. Maintaining the required low temperature can itself demand continuous external energy input.

The magnetic behavior has an important qualification. Type II superconductors permit magnetic flux to penetrate in quantized vortices over an intermediate range of applied fields. They can remain superconducting in that mixed state. Saying that every superconductor excludes every magnetic field under all conditions would miss this central feature of technologically useful materials.

A loss of superconductivity in a magnet is often called a quench. A region that becomes resistive can heat as current passes through it, so magnets need protection systems to manage stored energy. Zero resistance during normal operation does not eliminate the consequences of leaving the superconducting operating range.

Zero electrical resistance describes a particular state under suitable conditions, not a promise that a complete machine operates without energy input. Refrigeration, control electronics and changing magnetic conditions can still require power. The material property and the energy budget of an installation are separate questions.