Transformers are electrical devices that change or transform voltage levels between two circuits. In the process, current values are also transformed. However, the power transferred between the circuits is unchanged, except for a typically small loss that occurs in the process. Modern transformers differ considerably from these early models but the operating principle is still the same. In addition to transformers used in power systems, which range in size from small units that are attached to the tops of telephone poles to units as large as a small house and weighing hundreds of tons, there are a myriad of transformers used in the electronics industry. Transformers are fairly passive devices containing very few moving parts. Transformer windings and leads must operate at high voltages relative to the core, tank, and structural elements. In addition, different windings and even parts of the same winding operate at different voltages.