This mention statement is associated with
“In any network containing more than one sources of e.m.f. the current in any branch is the algebraic sum of a number of individual currents (the number being equal to the number of sources of e.m.f.), each of which is due to separate action of each source of e.m.f., taken order, when the remaining sources of e.m.f. are replaced by conductors, the resistances of which are equal to the internal resistances of the respective sources”.