This diagram appeared a hundred years ago this month in the May 1918 issue of Electrical Experimenter, and explains why users of field telephones in the trenches had to maintain security in their communications, even though there was no possibility that the line was tapped.

The diagram shows the Germans listening in on the Americans’ telephones, but it could just as easily be the other way around. By running a line parallel to the other side’s line, it was possible to pick up the conversation inductively. A powerful amplifier might be used, but in many cases, it was possible to listen in with an ordinary telephone receiver hooked to both ends of the line.