Telephone

A system that converts acoustic vibrations to electrical signals in order to transmit sound, typically voices, over a distance using wire or radio. For a long time, Alexander Graham Bell was considered the inventor of the telephone, along with Elisha Gray. However, Bell was not the inventor of this device, but only the first to patent it. This happened in 1876. On June 11, 2002, the Congress of the United States of America approved Resolution 269, in which it is recognized that the inventor of the telephone was Antonio Meucci, who called it telephone, and not Bell. In 1871 Meucci could only, due to financial difficulties, present a brief description of his invention, but not formalize the patent before the United States Patent Office. Although Meucci had made his first prototype in 1854, by 1861 Johann Philipp Reis had already developed another prototype independently, so even his patent application was late. The conventional telephone consists of two circuits that work together: the conversation circuit, which is the analog part, and the dialing circuit, which handles dialing and calling. Both voice signals and dial and call signals (signaling), as well as power, share the same pair of wires; This is sometimes called "signaling within the (voice) band. The characteristic impedance of the line is 600 Ω. The most striking thing is that the signals coming from the telephone to the central and those that are directed to it from it travel along that same line of only two threads. To be able to combine two signals (electromagnetic waves) in the same line that travel in opposite directions and then to separate them, a device called hybrid transformer or hybrid coil is used, which is nothing more than a power coupler (duplexer).

References:
 * 1) https://www.britannica.com/technology/telephone
 * 2) http://www.elon.edu/e-web/predictions/150/1870.xhtml