Absolute Zero and the Conquest of Cold  




Topic 8: Superconductivity

Superconductivity depends on the fact that as metal gets colder, electricity will flow through it more easily. For most metals, this is only true up to a point. Once they reach a certain temperature, there will always be some resistance to the flow of electrical current. But there are a few interesting metals -- superconductors -- where the resistance sharply drops to zero at some critical temperature. With the resistance gone, no voltage is required to keep the current moving -- and that means superconductors can deliver electricity far more efficiently than your average wire.

At the beginning of the 1900s, Heike Kamerlingh Onnes was an established Dutch physicist at Leiden University who had made great strides in creating extremely cold temperatures. In 1908, he was the first to liquefy helium, bringing the record man-made temperature down to -452 F (-269 C). Since he knew how to refrigerate materials to such incredibly cold temperatures, he decided to test how well metals could conduct electricity at those extremes. There were a lot of theories at the time. Some people, such as Lord Kelvin, believed that current would stop altogether. Onnes thought perhaps the resistance to the current in the material would gradually drop off and current might increase. (You can think of resistance as something like friction, it slows down electrical current much the way friction slows a sled on snow.)

Onnes studied a wire made of mercury, and measured the current at various cold temperatures. Initially, he noticed no change whatsoever -- he made the wire extremely cold, but at first it didn't affect the flow of electricity. Then, all of the sudden, once the wire hit 4.5 K, (-451.57 F or -268.65 C) all resistance disappeared. Current flowed through the wire and it didn’t need any voltage to keep moving. It would be as if someone gave you an initial push on a sled and even though you were on flat ground, the sled just kept going and going, never slowing down -- you could travel around the whole world without ever having to get another push. Onnes wrote: "Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state."

A magnet levitating above a "high-temperature" superconductor (with boiling liquid nitrogen underneath) demonstrates the Meissner effectIt took a while before scientists noticed a second important property of superconductors. In 1933, Karl Wilhelm Meissner and R. Oschenfeld discovered that superconductors are repelled by magnetic fields -- scientists say they are diamagnetic. So, not only are superconductors fantastic carriers of current, but they exhibit strange properties in the face of magnets. Two normal magnets cause each other to move -- you can feel a strong magnet practically jump out of your hand when you put it on the refrigerator. But a superconductor produces a stable magnetic effect -- a bar magnet will levitate easily and steadily over a superconductor, something which is called the Meissner effect to this day.

While scientists wanted to put the electrical and magnetic properties of superconductors to use in new technologies right away, the first superconductors had to be kept so cold in order to function that it just wasn’t practical. In the 1980s, scientists discovered materials that were superconducting at higher temperatures (77 K / -196 C / -321 F) -- though still incredibly cold by human standards. These materials can be cooled by liquid nitrogen, which is far easier to work with and much less expensive.

Additional Resources
More Activities

More Topics

  • Topic 1: Measuring the Cold - Thermometers
  • Topic 2: Understanding Heat and Energy
  • Topic 3: States of Matter
  • Topic 4: Refrigeration
  • Topic 5: Cryogenics
  • Topic 6: The Quest for Absolute Zero
  • Topic 7: How Animals Survive the Cold
  • Topic 8: Superconductivity
  • Topic 9: Astronomy
  • Topic 10: Spaceflight
  • Topic 11: Agriculture
  • Topic 12: Cold Medicine


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