(Lansing State Journal, Dec. 18, 1991)
Question submitted by P.W. Anderson of East Lansing.
Superconductivity describes a state that certain materials reach when cooled below a point called the critical temperature, Tc. Superconductivity is characterized by two main properties: absence of electrical resistivity and the Meissner effect.
Superconductivity was discovered in 1911 by Kammerlingh Onnes. Before 1986 the highest critical temperature of a material was 30 Kelvin (-406 degrees F). That year, A. Bednorz and K.A. Muller discovered certain types of materials can have critical temperatures as high as 125 Kelvin (-235 degrees F). Scientists are able to easily cool things to these temperatures, even though they are extremely low. Liquid nitrogen, which has a temperature of 77 Kelvin (-321 degrees F), is available and not very expensive. Cooling things below liquid nitrogen temperatures is much more expensive and not practical if the materials are to be used in everyday applications. Because the critical temperature of these new materials are so much higher than those previously known, scientist refer to these materials as `high Tc Superconductors'.
The first characteristic of a superconductor is that the resistively of the material goes to zero when the material is in the superconducting phase (i.e. cooled below Tc). In a normal material, the flow of electric current meets with resistance. The electrons which make up the electric current bounce off atoms and other electrons in the material, transforming part of the current into heat. This loss of current is not always desirable; the electric power companies lose a lot of money because of heat lost in the long lines which transport electricity from the generating plant to the consumer. Also, electric devices such as computer chips do not tolerate heat very well, and this intolerance places a limit on how small the chips can be made. Superconducting materials, which have zero resistance, do not generate heat and can therefore be used to solve these problems.
The second characteristic of superconductors is call the Meissner effect. Ordinary (non-superconducting) magnets have two poles, which we call north and south. Two south poles (or two north poles) will repel each other, while two unlike poles will attract. A superconductor will repel both north and south poles. This effect can be taken advantage of to levitate and propel trains. Such a train is already being tested in Japan.
The development of materials which become superconducting at higher Tc's promise to have continued impact on our lives. The two main characteristics of superconductivity - the lack of resistance and the Meissner effect - allow superconductors to replace normal materials and eliminate some of the problems which limit current technology.