The longitudinal magnetic field effect is caused by the characteristic flux motion driven by force-free torque. In this lecture, the peculiarity of this flux motion is discussed in association with the electric field induced. The reason why the variation principle cannot be used to derive the torque balance is also discussed. It is shown to be difficult to assume the flux cutting event as an explanation for various experimental results. Comparisons are performed relating to analogies in discovery between the theory of relativity and force-free torque from various aspects. Finally, the possibilities of superconducting power cables, the first application of the longitudinal magnetic-field effect, are introduced to show the enormous technological potential of this effect.
This introductory article describes the progress of magnetic separation technology including its background, principles, and application to process a large quantity of dilute suspension (i.e., up to 50 kg/hr). Development of this technology dates back to the 1970s when researchers at the Massachusetts Institute of Technology (MIT) invented "high gradient magnetic separation" (HGMS). The HGMS could enable magnetic separation to be applied to a large class of weak paramagnetic materials, down to colloidal particle size. The strength of the magnetic force generated with HGMS has since been increased by a factor of 103∼104 compared with that of permanent magnets. Due to the availability of direct, selective, high-intensity magnetic forces on particles to be separated, HGMS devices have been used for beneficiation of kaolin clay in the paper industry, for wastewater purification in the steel industry, and for recovery and recycling of glass grinding sludge in the glass fabrication industry. In 1986, Machine Design magazine reported that the first superconducting magnetic separator designed for commercial use was operating at the Huber clay-processing plant in Wrens, Georgia. Since then, a new 230-ton HGMS separator has been built by Eriez Magnetics. It requires fewer chemical additives, less space (34%), and weighs less (42%) than a conventional separator. However, due to retrofitting, only three separators of this type have been installed in the plant without any consideration for the generation of AC loss in superconducting wires. With development of technical R&D on applied superconductivity, two large-scale applications became commercially successful in the mid 90s; namely, magnetic separation and magnetic resonance imaging (MRI). At the time of February 1999, the number of HGMS systems in operation using superconducting magnets for magnetic separation was as follows: seven in Brazil, five in the U.S.A., four in the U.K., three in Germany, two in India, two in Australia, one in Austria, and one in Egypt. These systems were all equipped with a reciprocating canister HGMS in order to utilize superconducting persistent current mode, and were operated mainly in the kaolin clay beneficiation industry. Since 1995, the so-called "new magneto-science" has been extensively researched in Japan to study the effects of strong magnetic fields on non-magnetic (extremely weak magnetic) substances in a wide variety of scientific areas including solid state physics, chemistry, metallurgy, biology, and medicine. The initial driving force behind this research was the invention of a liquid helium-free superconducting magnet system in 1993, which could be more easily operated due to its use of high critical temperature superconducting materials developed since the 1986 discovery of cuprate superconducting matter. Magnetic separation is a primary application field for superconducting magnets, and in 1999, many HGMS application projects were initiated in Japan aiming at arsenic removal from geothermal water, purification of water seeping from reclaimed land, and purification of wastewater from paper factories. Magnetic seeding methods to various non-magnetic particles dispersed in water have been developed, such as electro-coagulation, electro-chemical oxidation, magnetic flock, and magnetic precipitation. Magnetic separation is also a promising technology to remediate soil polluted with radioactive particles.