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The telephone was invented by American A. G. Bell in 1876. He used two wires to connect two microphones and receivers with the same structure and a vibrating diaphragm on the electromagnet. However, the call distance is short and the efficiency is low. In 1878, a carbon microphone was introduced, which greatly improved the efficiency of the telephone microphone. The receiver structure has also been improved. The earliest telephone was a magnet phone, powered by its own battery, and the generator was used to send a call signal. In 1880, a common electric telephone was introduced, which was converted into a centralized power supply by a common electric switch, eliminating the need for a hand-powered generator and a dry battery. In 1891, a rotary dial-type automatic telephone was introduced, which can issue a DC dial pulse, control the automatic switch action, select the called user, and automatically complete the exchange function. Thereby pushing the telephone communication to a new stage. By the end of the 1960s, a push-button full electronic telephone was introduced. In addition to the pulse numbering method, a dual tone multi-frequency (DTMF) numbering method has appeared. With the development of program-controlled switches, dual-tone button telephones have gradually become popular. Electronic telephone circuits are moving toward integration, and telephone-specific integrated circuits have been widely used in various components of telephone circuits. Various multi-function telephones and special-purpose telephones have also emerged. By the early 1990s, there have been telephones that combine dialing, calling, and ringing on one integrated circuit. With the development of voice recognition technology, new telephones that use voice "dial" directly are also emerging.