The paper presents the foundations of a new theory of the two-winding autotransformer with obtaining the universal T-shaped equivalent circuit. The Universalism of the circuit is manifested in mapping the magnetic fluxes in the window and in the yoke of the autotransformer. This shows that the magnetic circuit of a short-circuited autotransformer experiences super – and antifluxes compared to the fluxes when idling. It is proved that these fluxes can be substantially higher than the respective fluxes in the short-circuited transformers.