A generalized theorem is given of which the well-known three-ray star-mesh transformation is a particular case. The formula is of a simple nature, and a great saving of labour results from its use. Three examples of application of the theorem follow: (1) To find the effective conductance between two points in a complicated network; (2) to find the effect of earth admittances on the balance of a Wheatstone bridge; and (3) to simplify the capacity network in the four-wire telephone cable and obtain the effective unbalance capacity.