Magnetic field levitation

It is perfectly diamagnetic, i.e. it completely excludes applied magnetic fields. This is the Meissner effect and is the reason why a superconductor can levitate a magnet. 

The Meissner Effect and Levitation. Besides the absence of electrical resistance, a superconducting material is characterized by perfect diamagnetism. The exclusion of magnetic field lines from a material when it passes from a normal state to a superconducting state is shown schematically in Figure 3. 

In addition to the zero resistivity, superconducting materials are perfectly diamagnetic in other words, magnetic fields (up to a limiting strength that decreases as the temperature rises toward Tc) cannot penetrate them (the Meissner effect). This is a consequence of the mobile, paired state of the electrons. Indeed, it is the demonstration of the Meissner effect, rather than lack of electrical resistivity, that is usually demanded as evidence of superconductive behavior. One entertaining consequence of the Meissner effect is that small but powerful magnets will float (levitate) above the surface of a flat, level superconductor.30... 

An interesting property of superconductors is their ability to create a mirror image of a magnetic field within themselves. As a result, they are repelled by magnetic fields and can even be levitated above them (see Fig. 1.53). This property has led to research on the possibility of levitating railroad trains and other vehicles fitted with superconductors over magnetic tracks (see Fig. 3.43). 

Fig. 13 Schematic representation of the orientation-dependent elastic torque on a nickel nanowire with longitudinal anchoring in 5CB (a) in equilibrium, and (b) in the presence of a magnetic field, (c) Levitation of the nanowire in a twisted nematic cell [446]. (Copyright 2004, American Association for the Advancement of Science)...

Figure 5.2 Scientists have created a means of revolutionary train travel. The "levitation" train travels over superconducting metal. This metal concentrates electricity so that it becomes an intense magnetic field. Magnetic parts in the train make the train levitate and travel at speeds close to 350 miles per hour.

Figure 6.28 Active magnetic bearing for high speed compressor drives. The rotor shaft is positioned in both the vertical and horizontal axes by actively controlled magnetic fields, generated by four levitation coils.

The levitation demonstration works only with Type II superconductors because the magnetic field lines that do enter the superconductor resist sideways motion and allow the balance of magnetic repulsion and gravitation to float the magnet above the superconductor. With Type I superconductors, the magnetic field lines cannot enter the superconductor at all and, because there is no resistance to sideways motion, the magnet will not remain stationary over the superconductor. 

In contrast to most levitation techniques such as levitation in electrostatic or magnetic fields, acoustic levitation requires no specific properties of the sample, so almost every substance, whether solid or liquid, can be acoustically levitated. The maximum possible diameter for a levitated sample is a function of the ultrasonic wavelength and turns out to be about one-half the wavelength under ambient atmospheric conditions. Usually, levi-tators are operated by ultrasound frequencies between 15 and 100 kHz resulting in wavelengths from 2.2 to 0.34 cm. 

Levitation As applied to electronic balances, the suspension of the pan of the balance in air by a magnetic field. 

There is good reason to be enthusiastic about these materials. Because liquid nitrogen is quite cheap (a liter of liquid nitrogen costs less than a liter of milk), we can envision electrical power being transmitted over hundreds of miles with no loss of energy. The levitation effect can be used to construct fast, quiet trains that glide over (but are not in contact with) the tracks. Superconductors can also be used to build superfast computers, called supercomputers, whose speeds are limited by how fast electric current flows. Furthermore, the enormous magnetic fields that can be created in superconductors will result in more powerful particle accelerators, efficient devices for nuclear fusion, and... 

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