During the first decade of the twentieth century, Kelley M. Turner of New York invented a telephone apparatus of very high sound sensitivity, which he called the “Dictograph.” (It should not be confused with the Dictaphone, a device used to record dictation.) Although his original idea was for a communications system with a great variety of applications, the Dictograph ultimately became one of the earliest electric eavesdropping devices, used by both police and private investigators. As such, the Dictograph played a part in some notable criminal prosecutions and was used in antiunion activity. It continued to be used in this way until it was rendered obsolescent by other technologies. The emergence of the preferred applications of the Dictograph illuminates aspects of the sociology of technology, such as the concept of “acoustic space.” It also raised issues related to the ethics and law of clandestine listening.