Fundamental circuits of 7e rotary telephone switching system

Continuation of a general description of the recently introduced 7E automatic system (see Abstr. 1957B03763 and 1957B05569) and the following fundamental circuits are shown and described: subscriber's line circuit, call detector, phase comparator which is one of the most original features of the system, group selector. Tests of calling and called lines are dicussed, as well as special features of P.B.X. lines. Design principles of 7E circuits include: wide operating limits for relays, avoidance of slow-operating relays and only limited usage of slow-releasing relays, non-homing switches and homogeneous grading.