The Inductophone

An interesting development of the telephone has recently been made by Mr Willoughby Smith, the well-known electrician. It has long been known that telephonic sounds could be transmitted through the air by means of "induction" from one wire to another, and, in the early days of the telephone, persons listening at a telephone connected to one telegraph line could overhear music and conversation being sent along a neighbouring wire. Mr Smith has devised an apparatus on this principle of induction, which enables a number of persons in the same room to hear telephonic sounds by simply holding a telephone to their ears in the neighbourhood of the apparatus in question, the influence being transmitted to the telephones through the air by induction. The inductophone, as the instrument is called consists of a wooden frame, in size and shape like an ordinary flat looking glass frame. The frame is wound across in a vertical direction with very fine platinum wire covered with silk, to prevent the different turns touching each other, and short-circuiting the current of electricity, which is received from the distant station and passed through the coil. This current, which conveys the sound, is in Mr. Smith's original apparatus transmitted by a vibrating tuning-fork, at the distant station. The tuning fork is placed in the circuit of a battery, and the line-wire, and at each vibration it sends a pulse of electricity into the line. These pulses traverse the wire of the inductophone at the receiving station, and in doing so produce a magnetic field around the coil which appears to extend to a considerable distance, and penetrates brick walls, glass, gutta-percha, and other non-magnetic substances. If now a telephone, which may have its coil removed and consist only of the iron diaphragm and magnet is held to the ear in the neighbourhood of the coil, the magnetic changes produced in the surrounding magnetic field by the intermittent current, circulating in the fine wire, will so affect the diaphragm that the sound of the tuning-fork will be clearly audible. The skeleton telephone should be held so that the iron diaphragm cuts the "lines of magnetic force" in the space around the coil, otherwise if it is parallel to these lines there will be no effect. While upon this subject, we may mention that a secret telephone company has been started in New York, whereby it is rendered very difficult for any one to overhear a message except the parties concerned. For this purpose two wires far apart are used, and the message is sent piecemeal over one or other wire, alternately, by means of a rapidly-rotating contact-maker. Any one tapping a single wire can thus only hear snatches of the message. For state and business uses this arrangement may be very serviceable.
Cassell's Family Magazine, 1883.