File:EB1922 Telephone - “Strowger” system diagram.jpg

English: “Strowger” system diagram (elementary circuit connexions): The calling subscriber on lifting his receiver operates relay “LR” which in turn operates “RR.” The impulses are received on “LR” whose armature falls back for each impulse placing an earth connexion intermittently on the circuit of “S” and “VM.” Both “RR” and “S” are sluggish relays that release slowly so that the succession of “breaks” and “makes” from the impulses do not affect them. When the long current between two trains of impulses occurs the circuit of “S” is disconnected for a period which is long enough for its operation, and “S” electromagnetically operates the switch “SS” so that “VM” is thrown out of circuit and “RM” is substituted. “VM” is the magnet which operates the vertical stepping by means of a ratchet and pawl device. “RM” similarly effects the rotary stepping. The next train of impulses will clearly operate “RM” after which the change-over devices depending on “S” are made to cut out the accessory mechanical apparatus, leaving only the connexions suitable for giving the engaged signal, ringing the subscriber, and talking. At the end of the conversation the caller hangs up his receiver, the armature of “LR” falls back permanently, “RR” is deenergized and the release magnet “REL” is operated thus restoring the switch to normal: “REL” disengages a detent which permits a coiled spring to restore the wipers or brushes in a rotary direction after which gravity carries them vertically to their home positions at which stage the “REL” circuit is opened at “ON.” The transmission system consists of two relays “LR” and “LC” associated with two condensers as shown at the top of the figure. This will be recognized as the “Stone” C. B. system.