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A superconductor is a material with zero electrical resistance, enabling efficient energy transfer without heat loss. Regular conductors like copper have resistance greater than zero but less than 0.01 ohms. When cooled below a critical temperature, superconductors transition into a state of absolute zero resistance, a phenomenon discovered by Heike Kamerlingh Onnes in 1911 with mercury.

The critical temperature is the threshold below which a material exhibits superconductivity with zero resistance. For mercury, this temperature is 4.2 Kelvin. Above the critical temperature, resistance changes linearly with temperature. Below it, the material enters a superconducting phase with no electrical resistance, making this transition point essential for identifying and utilizing superconducting materials.

The Meissner effect describes how magnetic flux is expelled from a superconductor when its temperature drops below the critical temperature, resulting in zero magnetic field inside the material. Above the critical temperature, the internal magnetic field equals the external field. This expulsion of magnetic fields is a defining characteristic of superconductors and demonstrates their unique interaction with electromagnetic fields.

The critical magnetic field is the minimum magnetic field strength that eliminates superconductivity in a material below its critical temperature. When this field is applied, the superconducting state is destroyed and the material returns to normal resistance. This parameter defines the operational limits of superconductors in practical applications involving strong magnetic environments.

Beyond mercury, niobium-nitride alloy became superconducting at 16 Kelvin in 1941, and vanadium-silicon at 17.5 Kelvin in 1953. Interestingly, excellent conductors like copper, silver, and gold do not exhibit superconductivity. Research continues to discover materials with higher critical temperatures, gradually expanding the range of superconducting substances available for technological applications.

The four-point method is necessary because superconductors have extremely small or zero resistance that standard ohmmeters cannot accurately measure. This specialized technique eliminates contact resistance errors by using separate current and voltage measurement paths, enabling precise detection of the near-zero resistance characteristic of superconducting materials.

Kamerlingh Onnes cooled mercury samples in liquid helium to study the relationship between temperature and resistance. As temperature decreased toward 4.2 Kelvin, resistance dropped linearly. At 4.2 K, resistance abruptly fell to zero, revealing the superconducting phase. This 1911 discovery established the foundation for understanding superconductivity and critical temperature phenomena.