To: E. Davies who wrote (2320 ) 5/18/2001 11:11:23 PM From: Bilow Read Replies (2) | Respond to Hi E. Davies; Re: " I have no claim to be an expert in superconductors, but I would assume that they still must have some finite resistance. If so the power dissapated in the wire generates heat. Finally- power is a function of voltage. "One could jump to the conclusion that the problem above a certain current density is localized heating- in that I'm just guessing. Do you know? " Since superconductors have no resistance, there is no heating, not even "localized" heating. If it was a matter of heating, it would have a more complicated form than Jc, as it would depend on how quickly heat could be conducted away from the superconductor. In other words, if there was a localized heating issue, the Jc factor would be lower for thicker superconductors than for thinner ones. If you don't trust my reasoning here, I'm happy to find links for you, but you can find them yourself with a few key strokes.In the big picture Ah is right- technology is nowhere close to economically sending power through superconducting wires. Thats probably many lifetimes away, and if someone said it's not then likely he's an idiot. "micro electronic circuitry where the watt per unit conduit cross section is small. ...Medical Museum - The Trail of Invisible Light: A Century of Medical Imaging ... For instance, Magnetic Resonance Imaging (MRI), perfected only in the 1980s, employs a superconducting magnet and computer-controlled radio-waves to generate images of inaccessible soft tissues; these images can be used to diagnose sports injuries, brain lesions, and spinal traumas. ... uihealthcare.com Englewood Hospital and Medical Center ... Our Open MRI is an advanced superconducting MRI that is open on all sides. This differs from the traditional form of MRI, which requires patients to lie inside an open-ended cylinder. ... englewoodhospital.com How Magnetic Resonance Imaging (MRI) Works ...There are three basic types of magnets used in MRI systems Resistive magnets consist of many windings or coils of wire wrapped around a cylinder or bore through which an electric current is passed. This causes a magnetic field to be generated. If the electricity is turned off, the magnetic field dies out. These magnets are lower in cost to construct than a superconducting magnet, but require huge amounts of electricity (up to 50 kilowatts ) to operate because of the natural resistance in the wire. To operate this type of magnet above about the 0.3 Tesla level would be prohibitively expensive.permanent magnet is just that - permanent. Its magnetic field is always there and always on full strength, so it costs nothing to maintain the field. The major drawback is that these magnets are extremely heavy - many, many tons in weight at the 0.4 Tesla level. A stronger field would require a magnet so heavy it would be difficult to construct . Permanant magnets are getting smaller, but are still limited to low field strengths.Superconducting magnets are by far the most commonly used . A superconducting magnet is somewhat similar to a resistive magnet - coils or windings of wire through which a current of electricity is passed create the magnetic field. The important difference is that the wire is continually bathed in liquid helium at 452.4 degrees below zero. Yes, when you are inside the MRI machine, you are surrounded by a substance that is that cold! Don't worry, it is very well insulated by a vacuum in a manner identical to that used in a vacuum flask . This almost unimaginable cold causes the resistance in the wire to be drop to zero, reducing the electrical requirement for the system dramatically and making it much more economical to operate. Superconductive systems are very expensive, but they can easily generate 0.5 Tesla to 2 Tesla fields, allowing for much higher quality imaging. howstuffworks.com google.com
