Superconductivity and energy conversion: A glimpse into the future

Nearly sixty years ago Kamerlingh Ohnnes (1) discovered a curious phenomenon on cooling mercury to liquid helium temperatures. Unlike the phenomena that occurs on cooling some materials, Ohnnes found that the electrical resistance of mercury not only decreased in value with temperature, but at some temperature point, which has become known as “critical temperature,” the mercury suddenly lost all of its resistance. Shortly thereafter a number of other metals were discovered to possess this interesting property. Examples of such metals are lead, tin, and tantalum. Most of these metals had temperatures less than 10°K. A great excitement ensued as a result of this discovery with many believing that at last a method of conducting great amounts of energy with no losses in power had been found. It was soon realized through experimentation that these metals could not sustain very large currents. Indeed instead of conducting an infinite current. when the value of the current exceeded some level of intensity, the superconducting metal became a normal conductor and because the currents were rather large. in some instances the metal vaporized entirely. A great deal of dismay propagated through the scientific community, who at that time were interested in this curious phenomenon. In 1933, Meissner and Ochsenfeld discovered that in addition to losing all of its resistance. a superconductor expelled all of its magnetic field, i.e. B = 0, from the superconducting interior.1 In 1930, de Haas and Voogd2 discovered that the Pb-Bi eutectic, which possesses the property of superconductivity, could sustain larger currents as well as larger fields. Indeed, the relation between currents and fields as expressed by Ampere's law, played an important role in explaining why superconductors were sensitive to currents beyond a certain critical value. It was learned that when the current that circulates through a superconductor exceeds a certain value, the resulting magnetic field repenetrates the material causing it to go normal. This value of the magnetic field is called the critical field. or more precisely, the thermodynamic critical field of the metal. denoted as a Type I superconductor.†