Some phases of railroad telegraph and telephone engineering

Based upon the idea of a duplex being a three-wire circuit, successful use has been made of a real line instead of the usual artificial line resistance box and condensers which are standard equipment in duplex sets. This “real” balancing line balances the regular line almost perfectly under changing weather conditions, needs no additional balancing at such times and hence can be designated “self-balancing.” It also balances for inductive disturbances to which both line and balancing line are exposed and for line irregularities such as short lengths of cable. This assumes that the duplex line and its balancing lines are alike in characteristics; are on the same pole line; have the same length; kind of wire, and, in fact, are twins. The two sides of a composite are very good for this type of set-up, but the plan is also applicable to any two similar circuits, such as straight wires or simplexes provided they are reasonably alike in kind.