English: A Tesla receiving device of the 1896-1899 structure illustrating a way of producing audible notes by reaction of the received impulses upon a magnetic field. On the left is a transmitter, diagrammatically represented, with an arrangement for varying the intensity of the waves emitted. The receiver on the left has a grounded antenna. The receiver secondary has a conductor under tension in a very powerful magnetic field. The reaction of this conductor, traversed by the received currents in the field, causes the conductor to emit audible notes. Top center shows an electromagnet of one form, employing a wire cord in the field, which, when current traverses it, vibrates and establishes a contact. At the bottom is a small coil through which current is passed, which by its vibrations produces an audible note or anything else desired.