The first transformer prototype

The first known prototype transformer was invented in 1831 by Michael Faraday. It consisted of two separate wire windings wound around a ring of soft iron, without a core. Faraday demonstrated that applying an alternating current in one winding created a changing magnetic field which, in turn, induced an electric current in the other winding. This was the first example of voltage and current transformation using an electric magnetic device. Later, in 1836, Nicholas Callan developed the first true transformer, which consisted of a single primary winding around an iron core and a remotely inductive second winding.

The prototype invented by Faraday, known as "ring", had limitations due to the heat produced by the excessive current and the inefficiency produced by the energy loss in the ring vacuum.
Later, the transformer was evolved to use a soft iron core to improve efficiency and reduce energy losses.