A device for transferring electrical energy from one alternating-current circuit to another with a change of voltage, current, phase, or impedance. It consists of a primary winding of Np turns magnetically linked by a ferromagnetic core or by proximity to the secondary winding of Ns turns. The turns ratio (Ns/Np) is approximately equal to Vs/Vp and to Ip/Is, where Vp and Ip are the voltage and current fed to the primary winding and Vs and Is are the voltage and current induced in the secondary winding, assuming that there are no power losses in the core. In practice, however, there are eddy-current and hysteresis losses in the core, incomplete magnetic linkage between the coils, and heating losses in the coils themselves. By the use of a laminated core and careful design, transformers with 98% efficiency can be achieved.