变压器的铁芯外侧为线圈，当交变电流通过线圈时，穿过铁芯的磁通量不断随时间变化，它在铁芯中产生感应电动势，从而产生涡流。这些涡流使铁芯大量发热，浪费大量电能，还可能损坏电器，严重时，甚至将铁芯发热局部熔化，发生铁芯“失火”。因此，我们要想办法减小涡流，途径之一便是增大铁芯材料的电阻率。
The outer side of the core of the transformer is a coil. When the alternating current passes through the coil, the magnetic flux passing through the core varies with time. It generates induction electromotive force in the core and produces eddy current. These vortices make iron core heat up, waste a lot of electric energy, and may also damage electrical appliances. Even when the core is heated, the core will be burned. So, we want to try to reduce the eddy current, one way is to increase the resistivity of the core material.