History of magnetism

Magnets are very common in our everyday environment and their existence has been known for centuries. The ancient Greeks (originally those near the city of Magnesia) and also the early Chinese had discovered strange and rare stones which had the power to attract iron. Around 1000, the Chinese even found that a steel needle could become magnetic when put into contact with one of these stones known as lodestone (magnetic iron ore). Furthermore, when freely suspended such a needle appeared to point north-south: in this way they invented the compass!

The magnetic compass then spread to western Europe and C. Colombus (1451 - 1506) was one of the first to make use of it while crossing the Atlantic in 1492. He even noticed that the needle was deviating from the north (as indicated from the stars) during the trip.

Later, around 1600, W. Gilbert (1544-1603), physician under Queen Elizabeth I of England, published De Magnete ("On the Magnet") which rapidly became a standard reference document on electrical and magnetic phenomena. For example, he was the first to make a clear distinction between magnetism and the amber effect (or static electricity as we call it today). Moreover, he linked the polarity of the magnet to the polarity of the Earth and built an entire magnetic philosophy on this analogy. According to Gilbert, magnetism was the soul of the Earth; a perfectly spherical lodestone would spin on its axis when aligned with the Earth's poles, just as the Earth spins on its axis in 24 hours... Although he didn’t express an opinion to whether this rotating Earth was at the centre of the universe or in orbit around the Sun, he was after N. Copernicus (1473-1543) one of those who inspired other famous scientists like J. Kepler (1571-1630) and later G. Galileo (1564-1642).

What is magnetism

Although everybody is familiar with the properties of magnets, one is generally much less aware of the origin of the magnetism. The concept of magnetism is based on the magnetic field or what is known as a dipole. The term magnetic field describes a volume of space where a detectable and measurable change in energy occurs. The poles are the locations where this field enters a material. A dipole is thus an object which has a magnetic pole at each end with opposite directions. Examples of this include bar or horseshoe magnets which both have a north and a south pole. The magnetic lines of force flow from pole to pole as shown in the sketch and it is easy to feel the attraction or repulsion when one plays with two magnets.

If a bar magnet is cut in two, both pieces remain magnetic. This sectioning with subsequent creation of dipoles is possible down to the atomic level, implying that the source of magnetism lies in the intrinsic properties of the atom itself. Since matter is composed of atoms, all materials are sensitive in some way to magnetic fields. Without going into detail, the source of magnetism lies in the field created by the motion of electrons within the atoms.

Electromagnetic fields

Magnets are not the sole source of magnetism. As previously stated, a flow of electrical charge (for example an electric current flowing in a wire) causes a compass positioned nearby to deflect. This was the experiment performed by H.C. Oersted in 1820 which demonstrated the possibility to produce magnetic fields: these fields are known as electromagnetic fields. Oersted also noticed that the field strength is proportional to the electrical current within the wire and that the field direction depends on the direction of the current.

Since this discovery much effort has been applied to develop different designs in order to produce increasing magnetic fields. The figure below illustrates a wound coil which produces an uniform magnetic field in its center with a strength not only proportional to the current but also the number of loops in the coil.


Why using superconductors for high magnetic fields applications

Electromagnetic fields produced with conventional conductors are used almost everywhere in today's technological environment, in particular in electric motors, transformers or power generation systems. With cooling costs being less and less a limitation, the development of energy efficient devices using superconductors is very promising.

Superconductors are used today in various kinds of applications using high magnetic fields such as Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance spectroscopy (NMR). Other applications are under development: high current generators, current leads, energy transfer superconducting cables, fault current limiters, motors, transformers, energy storage systems and also electronic devices. Since superconducting wires are very efficient conductors (being able to carry much higher current densities than normal conductors), it is possible to design coils made from such materials which can produce very high, stable and uniform magnetic fields. Moreover, superconducting generators half the size of a copper wire generator are about 99% efficient; conventional generators are around 50% efficient.

Four spectacular applications which make use of superconducting magnetic fields are illustrated below.

For more information on high magnetic fields produced using superconductors:
Prof. R. Flükiger team
Bruker Biospin

Magnetic field strength

Three different types of magnets exist: permanent, superconducting, and pulsed magnets.

Neodymium-iron-boron and ternary samarium-cobalt alloys are currently the strongest known permanent magnets and can produce fields of about 1 Tesla (the Tesla T is the international unit of magnetic field strength: one Tesla is about 20'000x the earth magnetic field).

Superconducting magnets are a kind of electromagnet producing a magnetic field from the flow of electric current through a material with no resistance. A superconducting magnet can reach field strengths above 20 T.

Finally, pulse magnets can provide even higher magnetic field up to 72 T, but their use is limited to research laboratories.

Superconductivity and applications: some examples

• Magnetic levitation
  
  The so-called "MagLev" trains such as the Yamanashi train have been under development in Japan for the past two decades. The train levitates above a track using superconducting magnets, with the advantage of eliminating friction. This allowed to reach very high speeds up to 552 Km/h (April 14, 1999).
  The Yamanashi MLX01 test line in Japan is 19 Km long.
  
  
• Medical imaging
  
  Magnetic Resonance Imaging (MRI) has been up to now the most successful application of superconductors. This medical imaging technique provides very precise diagnostics with high spatial resolution (without any surgical intervention on the patient). 2 T systems are presently installed worldwide and progress is going on: an operational full body MRI system reaching 7 T has recently been setup…more about magnetic field strength.
  
  
• High resolution spectroscopy
  

  NMR Magnet 900 MHz, 21 T

  Similar to MRI, Nuclear Magnetic Resonance spectroscopy (NMR) is probably the most common technique used to determine detailed molecular structures in chemical industry and in biotechnology, as for example genetic material (DNA and RNA) and other complex molecules used to develop new drugs. With the advent of molecular biology the demand for spectrometers with increasingly high resolution would not be satisfied without superconducting magnets.
  
  
• Ship propulsion motor
  

  5MW Rotor Assembly with Exciter. (Courtesy of American Superconductors)

  The American Superconductors Corporation is developing 5MW ship propulsion motors using high temperature superconductors (under test summer 2003). This technology presents the following advantages: inherently quieter motors, 25% of the volume of a standard motor, lower operating cost, 30% of the weight, equivalent prices and higher net efficiency.