To concentrate the magnetic field in an electromagnet, the wire is wound into a coil many times, ensuring that the turns wire are side by side along the edge. The magnetic field generated by the turns of wire passes through the center of the coil, creating a strong magnetic field there.
The side of the magnet that the field lines emerge from is defined to be the north pole. However, much stronger magnetic fields can be produced if a ferromagnetic material i. These wires are also kept at cryogenic temperatures to ensure that electrical resistance is minimal. Such electromagnets can conduct much larger currents than ordinary wire, creating the strongest magnetic fields of any electromagnet, while also being cheaper to operate because of there being no energy loss.
Today, there are countless applications for electromagnets, ranging from large-scale industrial machinery, to small-scale electronic components. In addition, electromagnets are used extensively for the sake of conducting scientific research and experiments, especially where superconductivity and rapid acceleration is called for.
In the case of electromagnetic solenoids, they are used wherever a uniform i. As a result, solenoid magnets are to be commonly found in electronic paintball markers, pinball machines, dot matrix printers and fuel injectors, where magnetism is applied and controlled to ensure the controlled movement of specific components. Given their ability to generate very powerful magnetic fields, low resistance, and high efficiency, superconducting electromagnets are often found in scientific and medical equipment.
Electromagnets are also used extensively when it comes to musical equipment. These include loudspeakers, earphones, electric bells, and magnetic recording and data storage equipment — such as tape recorders. The multimedia and entertainment industry relies on electromagnets to create devices and components, such as VCRs, and hard disks. Electrical actuators, which are motors responsible for converting electrical energy into mechanical torque, also rely on electromagnets.
Electromagnetic induction is also the means through which power transformers function, which are responsible for increasing or decreasing the voltages of alternating current along power lines. Induction heating, which is used for cooking, manufacturing, and medical treatment, also relied on electromagnets, which convert electrical current into heat energy.
In its deliberations, the committee took as its purview both the disciplines relevant to the generation of high magnetic fields and those that would benefit if higher fields could be generated. In many instances, the quantity that determines whether a magnet is high field is the amount of energy stored in its field, which is proportional to the integral of the square of its field strength over the volume affected. Thus, a magnet having a maximum field strength around 8 T and a bore large enough to accommodate a human being is as much a high-field magnet as the much smaller bore magnet in a nuclear magnetic resonance NMR spectrometer operating at 20 T.
Some magnets operate in a pulsed mode, which alleviates some of the constraints that limit the fields achievable by DC magnets. See Box 1. While this definition of high field will not help the reader decide whether a magnet of one type operating at field x is a higher field magnet than a magnet of anther type operating at field y , it does make clear why it is difficult to increase the maximum field strength delivered by magnets of any given type.
The materials in a high-field magnet of any given type are, by definition, close to failure. As already noted, the construction of high-field magnets has always posed engineering challenges. Solenoids generating fields of about 2 T were built in the 19th century using resistive conductors, and even at fields that low, both the mechanical strength of the materials used and heating were issues. In the s, W.
Giauque and F. Bitter built water-cooled magnets of novel design from. The magnets in clinical medical MRI spectrometers operate around 4 T; the current highest field is 9. Strongest pulsed magnetic field ever achieved with explosives in the laboratory Sarov, Russia , 2, T. Maximum theoretical field strength for a neutron star, and therefore for any known phenomenon, is 10 13 T.
Bitter magnets are still used today. Modern versions produce fields of T. While they are relatively inexpensive to build tens of thousands of dollars , they are costly to operate because of the power they consume and the cooling they require.
At fields in this range the energies stored in a magnet of useful size are so large that mechanical failure can have dangerous consequences. Onnes discovered that many metals become superconducting at temperatures close to 0 K. For magnet designers, superconductivity looked like a godsend.
The flow of current through the coils of a superconducting electromagnet generates no heat because there is no resistance, so no cooling is required beyond that needed to maintain its coils in the superconducting state. In addition, once energized, superconducting magnets consume no power and do not have to be connected permanently to a power supply. However, it was soon discovered that no matter how cold they are, the metals in which superconductivity was first demonstrated become resistive when exposed to magnetic fields much lower than those generated by the resistive electromagnets of the day.
The quenching of the superconducting state by external magnetic fields occurs in all superconducting materials, not just the metals studied by Onnes. What varies from one super-. Only in were materials discovered that remain superconducting in fields high enough to be interesting to magnet designers, and the use of these materials for magnet fabrication has exploded since then.
The number of superconducting magnets operating in instruments in laboratories and hospitals around the world is hard to estimate, but the committee was told by an industry representative that every year manufacturers sell about 2, MRI instruments and roughly NMR spectrometers. These instruments contain superconducting magnets collectively worth billions of dollars. Other arenas in which superconducting magnets are used on a large scale are high-energy physics and fusion research. As is explained in the body of this report, the construction of magnets from superconducting wire is a complex art.
The performance of all such magnets is limited by the properties of the superconductors from which they are made, especially their critical fields. Mechanical strength and fabricability are also vital issues. These challenges notwithstanding, the maximum strengths of the fields produced by superconducting magnets have gradually increased to about 25 T.
Hybrid magnets, which consist of a resistive solenoid inside a superconducting solenoid, can deliver substantially higher DC magnetic fields about 45 T , but of course they continuously consume power and generate heat in their normal conducting sections. In , materials were discovered that superconduct at temperatures up to K, much higher than the highest temperature achieved by previously known superconducting materials about 23 K.
These high-temperature superconductors are ceramic copper oxides, which suffer from intrinsically weak links at internal grain boundaries, making the fabrication of magnets from them extremely difficult. They are very interesting to magnet designers, however, because their critical fields are far higher than those of any of the superconductors now routinely used for magnet fabrication.
The technical challenges they pose are being overcome, so the field strengths that can be obtained from superconducting magnets are likely to increase significantly in the next few years. Resistive magnets can generate fields with strengths greater than about 45 T, but only for short times. If partial or total instrument destruction can be tolerated, fields well. As the duration of the field pulse a magnet delivers declines, however, so too does its utility as a tool for scientific research.
Consequently, the committee took the view that both the technologies and the science associated with fields of very short duration less than a few milliseconds lie outside the scope of its inquiry. When the field was downward oriented and swept counterclockwise, the scientists observed a significant decrease in the alpha wave amplitude when they pooled the data from 26 subjects for analysis. But not all conditions elicited the change.
Nor did they see a response when the field was pointed down and rotated clockwise. The question is, how does it work? It makes sense, as magnetic sensory systems seem to occur in virtually all organisms, says Kirschvink.
Both Winklhofer and Kirschvink point to Aboriginal peoples in Australia who are known for their ability to orient in the desert and whose language references cardinal directions North, South, East, and West rather than relative ones right, left, forward, and backward.
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