In a transformer, the coil across which the input voltage is applied is called the:

1. A transformer has two separate wire coils wound on a shared magnetic core. 2. The text specifies their roles: "the input voltage is applied across the primary coil, and the secondary produces the transformed output voltage." 3. So the coil receiving the input voltage is, by definition, the primary coil, while the output appears on the secondary coil. 4. "Core coil" and "induction coil" are not the terms the text uses for either winding. _Source: OpenStax Physics Ch 20 "Magnetism", p.690 §20.2 Motors, Generators, and Transformers_