These magnets can be permanent magnets or electromagnets. Superconducting magnets have a number of advantages over resistive electromagnets. Magnetic resistance models control resistance by means of permanent magnets or electromagnets. Electromagnets with fixed cores are not considered solenoids. Superconducting magnets are some of the most powerful electromagnets known. In 1914, utilized alternating electromagnets pulling on an iron armature. Forks can be driven electrically by placing electronic oscillator-driven electromagnets close to the prongs. "Soft" magnets (low coercivity) are used as cores in transformers and electromagnets. These devices are known as destructive pulsed electromagnets. Magnetically "soft" (low coercivity) iron is used for the cores in electromagnets. In comparison Transrapid uses conventional electromagnets and attractive-type electromagnetic suspension (EMS). The self-excited magnetic field dynamo did away with the battery to power electromagnets. Stepper motors effectively have multiple "toothed" electromagnets arranged around a central gear-shaped piece of iron. Thus, management of magnetic pressure is a significant challenge in the design of ultrastrong electromagnets. The technique enables the levitation of an object using electromagnets.