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A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays". ==History== The American scientist Joseph Henry is often claimed to have invented a relay in 1835 in order to improve his version of the electrical telegraph, developed earlier in 1831.〔 〕〔 (【引用サイトリンク】publisher=Georgi Dalakov ) 〕〔 〕〔 〕 However, there is little in the way of official documentation to suggest he had made the discovery prior to 1837.〔https://books.google.co.uk/books?ei=xPkZVZ3BFNbWavGogpAO&id=xjUhAQAAIAAJ&dq=Early+Electrical+Communication&focus=searchwithinvolume&q=Henry+Relay〕 It is claimed that the English inventor Edward Davy ''"certainly invented the electric relay"'' in his electric telegraph c.1835. A simple device, which we now call a relay, was included in the original 1840 telegraph patent〔(US Patent 1,647, Improvement in the mode of communicating information by signals by the application of electro-magnetism, June 20, 1840 )〕 of Samuel Morse. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing signals to be propagated as far as desired. This overcame the problem of limited range of earlier telegraphy schemes. The word ''relay'' appears in the context of electromagnetic operations from 1860.〔 http://www.etymonline.com/index.php?allowed_in_frame=0&search=relay&searchmode=none 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Relay」の詳細全文を読む スポンサード リンク
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