A telephone, or telephone, is a telecommunications device that allows two or more users to hold a conversation when they are too far away to be heard directly. A telephone converts the sound, typically and more efficiently the human voice, into electronic signals that are transmitted through cables and other communication channels to another telephone that reproduces the sound to the receiving user.
In 1876, Scottish emigrant Alexander Graham Bell was the first to obtain a United States patent for a device that produced a clearly intelligible replication of the human voice. This instrument was developed by many others. The telephone was the first device in history that allowed people to talk directly to each other at great distances. Telephones quickly became indispensable for businesses, government and homes, and today are some of the most used small appliances.
The essential elements of a telephone are a microphone (transmitter) to speak and an earpiece (receiver) that reproduces the voice in a distant place. In addition, most phones contain a buzzer to announce an incoming phone call, and a dial or keypad to enter a telephone number when initiating a call to another telephone. The receiver and transmitter are usually integrated into a headset that is placed on the ear and mouth during conversation. The dial can be located on the headset or on a base unit to which the headset is connected. The transmitter converts the sound waves into electrical signals that are sent through a telephone network to the receiving telephone, which converts the signals into an audible sound in the receiver or, sometimes, in a loudspeaker. Telephones are duplex devices, which means they allow transmission in both directions simultaneously.
The first telephones were directly connected to each other from the office or residence of a client to the location of another client. Being impractical beyond a few customers, these systems were quickly replaced by manually operated central distribution boards. This gave rise to the fixed telephone service in which each telephone is connected by a pair of cables dedicated to a switching system of the local central office, which became fully automated systems from the beginning of 1900 and, finally, in a switched global public telephone network. For greater mobility, several radio systems were developed for transmission between mobile stations on ships and cars in the mid-twentieth century. Hand-held mobile phones were introduced for personal service as of 1973. In later decades, their analogue cellular system evolved into digital networks with greater capacity and lower cost.
Convergence has given most of the capabilities of cell phones more than just a simple voice conversation. It is possible that they can record spoken messages, send and receive text messages, take and show photographs or videos, play music or games, surf the Internet, navigate the road or submerge the user in virtual reality. Since 1999, the trend of mobile phones is that smartphones integrate all the needs of communication and mobile computing.