6. Note on the Wire Microphone

Abstract

At our last meeting Professor Chrystal showed us that a fine platinum wire attached to a stretched disc of skin could act as an electric telephone receiver for the sounds of a violin. The wire was included in a galvanic circuit, and the variations of current were made by a microphone attached to the violin. The account he gave of this interesting experiment was that the receiving wire became extended by the heat of the current either as it was established or suddenly increased by the microphone, and correspondingly shortened on the current ceasing. These extensions and contractions were rendered audible by the disc. A similar demonstration with a like commentary was made by Mr Preece to the Royal Society of London, an account of which was published in “Nature” (June 10). Mr Preece got his wires to speak. At the first May meeting of this Society in 1878 I discussed the subject of the sounds emitted by fine wires, giving passage to intermittent currents. I found that the ordinary thread telephone gave us an easy means of hearing these sounds in non-magnetic metals. De la Rive had heard them in 1845, but since his time no one had been able to hear them, and they were almost looked on as apocryphal. I attached the thread of the skin or paper telephone transversely to the sounding wire, and not directly, as Professor Chrystal has done, for the simple reason that I found that the transverse method gave equally good results with very much less trouble. The cause in both cases seemed to me the same, viz., an internal molecular click which marked the setting in and stoppage of the current.