Often referred to as a bearing heater, the mains frequency unit employs standard transformer principles for its operation. An internal winding is wound around a laminated core similar to a standard mains transformer. The core is then passed through the work-piece and when the primary coil is energised, a magnetic flux is created around the core. The work-piece acts as a short circuit secondary of the transformer created, and due to the laws of induction, a current flows in the work-piece and heat is generated. The core is normally hinged or clamped in some way to allow loading or unloading, which is usually a manual operation. To cover variations in part diameter, the majority of units will have spare cores available which help to optimise performance. Once the part is heated to the correct temperature, assembly can take place either by hand or in the relevant jig or machine press.