<P> Usually ferrite or similar . This is used for inductors and transformers . The shape of a pot core is round with an internal hollow that almost completely encloses the coil . Usually a pot core is made in two halves which fit together around a coil former (bobbin). This design of core has a shielding effect, preventing radiation and reducing electromagnetic interference . </P> <P> This design is based on a toroid (the same shape as a doughnut). The coil is wound through the hole in the torus and around the outside . An ideal coil is distributed evenly all around the circumference of the torus . The symmetry of this geometry creates a magnetic field of circular loops inside the core, and the lack of sharp bends will constrain virtually all of the field to the core material . This not only makes a highly efficient transformer, but also reduces the electromagnetic interference radiated by the coil . </P> <P> It is popular for applications where the desirable features are: high specific power per mass and volume, low mains hum, and minimal electromagnetic interference . One such application is the power supply for a hi - fi audio amplifier . The main drawback that limits their use for general purpose applications is the inherent difficulty of winding wire through the center of a torus . </P> <P> Unlike a split core (a core made of two elements, like a pair of E cores), specialized machinery is required for automated winding of a toroidal core . Toroids have less audible noise, such as mains hum, because the magnetic forces do not exert bending moment on the core . The core is only in compression or tension, and the circular shape is more stable mechanically . </P>

What is the purpose of the soft iron core in a transformer