One of the most important properties of a class of materials called "superconductors" is that magnetic fields cannot penetrate into the material. This means they can be used to levitate magnets and even to build magnetic levitation trains (this is currently being done in Japan), which float 10mm above the tracks on a magnetic field. Some examples of superconductors that people have know about for over 90 years are Mercury, Tin, and Lead.

The only problem with these materials is that they must be at a temperature of less than -270 degrees Celsius to work as superconductors! More recently people are starting to discover "high-temperature" superconductors, which work at temperatures as high as -110 degrees Celsius (which still sounds pretty darn cold to the average person...).

Yes- magnetic fields can't penetrate a superconductor. They can't even pass through the hole in a loop made of superconductor.

One of the basic laws of physics is that any changing magnetic field creates an electric field, which exerts forces on charged particles such as electrons. Voltage is a measurement of electric field strength, and as you know, applying voltage to a conductor causes electrons to move. This is electricity.

So, a changing magnetic field in a conductor creates electricity in the conductor. The effect even works when a conductor (such as a loop of wire) encircles a magnetic field.

This is how generators work- rotating magnets spin next to or inside loops of wire, and electricity comes out of the wires. This is also why you can fry sensitive electronic devices by swiping a magnet past them. This even explains why solar storms can cause blackouts.

Hot gas and charged particles from the Sun change the shape of the Earth's magnetic field. This field is weak, but the gaps between major power lines can be hundreds of miles across. The bigger the loop, the higher the voltage- so even a small change in Earth's weak field can cause voltage surges which shut down the entire grid.

Now on to superconductors.Superconductors are materials which conduct electricity without any loss or dissipation- obviously a useful property! Some common metals (such as lead) can become superconductors, but all known superconductors lose their superconducting ability if they're not kept extremely cold, which is why you don't see them used everywhere.

In a loop of superconductor, electricity can even run around chasing its own tail- forever. This would never happen in an ordinary conductor because of the problem of maintaining a voltage, but superconductors don't need voltage to conduct.

Ohm's Law states that
voltage=current*resistance, or V=I*R. In superconductors R = 0 and I cannot be infinite, so therefore V must also be 0. If you put a voltage on a superconductor, electricity will always flow in a way which cancels out that voltage.

A changing magnetic field generates voltage, so the electricity must flow in a way which cancels the change in the magnetic field. You know from the electromagnet that electricity creates a magnetic field. The superconductor automatically creates the right pattern of electricity to generate a counter-field which cancels any change in the magnetic field piercing the superconductor!

Now, if the superconductor becomes superconducting without any magnetic field present, no field will be able to pierce it as long as it remains superconducting. Put the superconductor over a magnet, and its counter- field can even become strong enough to levitate the superconductor by magnetic repulsion. That's why you see those pictures of the superconductor floating in a cloud of mist.

Conversely, if the superconductor is cooled below its critical temperature while a magnetic field is present, the superconductor will retain that field unchanged. If the original magnet is removed, the superconductor just becomes an electromagnet and regenerates the field.

Such interesting magnetic effects aren't foolproof, though. High magnetic fields induce high electric currents, which will remove the superconductor's superpowers just as surely as high temperatures will. So, a super-strong magnetic field will penetrate a superconductor, but first it must make it lose its superconductivity.

Super-cool temperatures can also transform helium into a superfluid. If your orange juice was a superfluid, it would crawl up the side of your glass and escape. The rare mineral magnetite has its own magnetic field, so when it's in a repulsive configuration it can keep out a magnetic field weaker than its own. This is not a perfect shield like a superconductor, because inside the magnetite the influence of the opposing field can still be felt.

Iron and niobium can also become magnetized, so they can keep out magnetic fields in the same way magnetite does. Iron and niobium can also distort and even amplify magnetic fields, but the effects depend on the shape of the metal. You can make an electromagnet by winding wire around a stick, but an effective electromagnet uses a steel nail as a magnetic amplifier. The nail not only allows the field to pass through it, but it also makes it stronger. However, a large steel plate seems to block magnetic fields completely.