This chapter discusses the first used 'transformer', called an induction coil which is used in motor vehicles to raise the DC motor, provided by a lead-acid battery. An electrical machine transfers the electrical energy from one circuit to another circuit through inductively coupled conductors. In the electricity industry, coils are termed as 'windings'. The primary windings and secondary windings of the transformer are made from insulated copper conductors which is a rectangular cross-sectional area that carries the transformer's primary and secondary currents without overheating. Isolation transformers are designed and used for isolation purposes. The types of transformer are classified as follows: distribution transformers, isolation transformers, power-supply transformers, instrument transformers are correctly known as 'mutual transformers'. Mutual transformers are sometimes called 'constant-voltage transformers', which means there is no significant difference between their 'no-load' voltage and their 'full-load' voltage. The primary and secondary windings of all mutual transformers are electrically isolated from each other by a common magnetic field via silicon-steel cores.