Mid 19th Century global telegraph lines. Map of the world with coloured lines depicting the stages at which people were laying down cables for electromagnetic telegraph links. The green lines show the cables in operation, the red ones show those under contract and the blue lines show the contemplated telegraph links. These cables transmit messages in the form of electrical signals around the world. Electric telegraphs were an important application of the newly-discovered electromagnetic force. The first practical telegraph was invented by Samuel Morse in 1837. They quickly spread across the USA and Europe, and revolutionised communications. The two continents were permanently linked in 1866 by an underwater cable crossing the Atlantic Ocean.