Induced current in parallel circuits and its effect upon relays

MUTUAL INDUCTION is very much like friction in that it is a great blessing under certain conditions and an unmitigated nuisance under others. If it were not for mutual induction there would be no a-c systems as we know them today because there would be no transformers. It is apparent, therefore, that mutual induction is the basis of an entire industry that could not exist without it. On the other hand it has been the bane of the communication industry where induction between circuits is highly undesirable. There is a story told about one of the early long-distance open-wire telephone circuits having several parallel lines in which the experimenter at one end spoke to the man at the other end and asked “Do you hear me?” The reply was, “Perfectly.” The first man then asked the second, “Which line am I on?” and after a moment's hesitation the second replied, “All of them.” These two illustrations show that mutual induction may be either a blessing or a curse depending upon the circumstances under which it exists.