<Li> Hysteresis losses: Reversing the direction of magnetization of the magnetic domains in the core material each cycle causes energy loss, because of the coercivity of the material . These losses are called hysteresis . The energy lost per cycle is proportional to the area of the hysteresis loop in the BH graph . To minimize this loss, magnetic cores used in transformers and other AC electromagnets are made of "soft" low coercivity materials, such as silicon steel or soft ferrite . </Li> <P> The energy loss per cycle of the AC current is constant for each of these processes, so the power loss increases linearly with frequency . </P> <P> When a magnetic field higher than the ferromagnetic limit of 1.6 T is needed, superconducting electromagnets can be used . Instead of using ferromagnetic materials, these use superconducting windings cooled with liquid helium, which conduct current without electrical resistance . These allow enormous currents to flow, which generate intense magnetic fields . Superconducting magnets are limited by the field strength at which the winding material ceases to be superconducting . Current designs are limited to 10--20 T, with the current (2009) record of 33.8 T . The necessary refrigeration equipment and cryostat make them much more expensive than ordinary electromagnets . However, in high power applications this can be offset by lower operating costs, since after startup no power is required for the windings, since no energy is lost to ohmic heating . They are used in particle accelerators and MRI machines . </P> <P> Both iron - core and superconducting electromagnets have limits to the field they can produce . Therefore, the most powerful man - made magnetic fields have been generated by air - core nonsuperconducting electromagnets of a design invented by Francis Bitter in 1933, called Bitter electromagnets . Instead of wire windings, a Bitter magnet consists of a solenoid made of a stack of conducting disks, arranged so that the current moves in a helical path through them, with a hole through the center where the maximum field is created . This design has the mechanical strength to withstand the extreme Lorentz forces of the field, which increase with B . The disks are pierced with holes through which cooling water passes to carry away the heat caused by the high current . The strongest continuous field achieved solely with a resistive magnet is 37.5 T as of 31 March 2014, produced by a Bitter electromagnet at the Radboud University High Field Magnet Laboratory in Nijmegen, Holland . The previous record was 35 T . The strongest continuous magnetic field overall, 45 T, was achieved in June 2000 with a hybrid device consisting of a Bitter magnet inside a superconducting magnet . </P>

What is the function of the switch in the electromagnet