Transformers need an "amplifier" for the magnetic field so that the number of winding turns can be kept low. This "amplifier" is the so-called magnetic core. It consists of ferromagnetic iron, which contains microscopic elementary magnets that align to the transformer's magnetic field as a compass needle aligns to the Earth's magnetic field. The iron core is made of many thin ferromagnetic steel sheets that are electrically insulated against each other and stacked. This reduces classical eddy losses. The use of special alloys and manufacturing methods enables a minimum needed energy to change polarity of the elementary magnets.
