This book introduces readers to electric circuits with variable loads and voltage regulators. It defines invariant relationships for numerous parameters, and proves the concepts characterizing these circuits. Moreover, the book presents the fundamentals of electric circuits and develops circuit theorems, while also familiarizing readers with generalized equivalent circuits and using projective geometry to interpret changes in operating regime parameters. It provides useful expressions for normalized regime parameters and changes in them, as well as convenient formulas for calculating currents. This updated and extended third edition features new chapters on the use of invariant properties in two-port circuits, invariant energy characteristics for limited single-valued two-port circuits, and on testing projective coordinates. Given its novel geometrical approach to real electrical circuits, the book offers a valuable guide for engineers, researchers, and graduate students who are interested in basic electric circuit theory and the regulation and monitoring of power supply systems.