Though the invention came as the result of several decades of research by many people, Samuel F. B. Morse (1791–1872) is credited with making the first practical telegraph, the first instrument that could send messages across wires via electricity, in 1837. Morse was a portrait painter in Boston when he became interested in magnetic telegraphy in about 1832. With technical assistance from chemistry professor Leonard Gale (1800–1883) and financial support from Alfred Vail (1807–1859), Morse conducted further experiments. He also developed Morse code, a system of variously arranged dots and dashes, which can be used to transmit messages. (For example, the most frequently used letter of the alphabet is e, which is rendered in Morse code by using one dot; the less frequently used z is rendered by two dashes followed by two dots.) By 1837 Morse had demonstrated the telegraph to the public in New York, Philadelphia, and Washington, D.C. He received a patent for his invention in the United States in 1840. In 1843 his invention got a boost when the U.S. Congress approved an experimental line, to be built between Washington, D.C., and Baltimore, Maryland. The following year, on May 24, 1844, Morse sent his first message across that line: “What hath God wrought!” Vail was on the receiving end of the wire.

By 1861 most major U.S. cities were linked by telegraph wires. The first successful transatlantic cables were laid in 1866. Morse code transmissions, called telegraphs when transmitted via aboveground wires and cablegrams (or cables) when transmitted via underwater cables, were translated by operators or mechanical printers on both the sending and receiving ends of the message. The introduction of the telegraph marked the beginning of modern communications: When the first transcontinental telegraph line in the United States was completed on October 24, 1861, it eliminated the need for the Pony Express, which had briefly enjoyed the status of the fastest way to transmit a message—about eight days from St. Louis, Missouri, to Sacramento, California, a distance that could be bridged by telegraph lines within minutes. The telegraph became the chief means of long-distance communication. The telephone (invented 1875), which allows voice transmission over electrical wires, gradually replaced the telegraph. But for many decades the two technologies were both in use.