Overland telegraph technology implicitly embodied a familiar image of electricity as being a mobile substance confined to conductors.  But phenomena encountered in undersea telegraph cables presented a competing image, one far more suggestive of Faraday’s image of tension in a dielectric.  Subsequent production of electromagnetic waves could be seen as the ultimate practical expression of Faraday’s “tension” image.  But if they were to serve as a communications medium, electromagnetic waves had not only to be produced but be detected; and although various detecting mechanisms were developed, their operation was mysterious: the received images of electric current appeared to provide little clarity.