Why does the letter E get the shortest signal in Morse code?

Because E is the most common letter in English. Morse code turns text into two signal lengths, short dits and long dahs, and Alfred Vail, the engineer working with Samuel Morse, designed the alphabetic code so that each letter's duration was roughly inverse to how often it appears in English text. The letter used most often costs the least time to send: E is a single dit. Later revisions made a few codes less than optimal, but many still follow the rule.

The timing is strict. A dah lasts three times as long as a dit, each signal inside a letter is followed by a silence one dit long, letters are separated by three dits of silence and words by seven. The code carries Morse's name, but several people built it: Morse's own first proposal was replaced by Vail's alphabet-based code, used for commercial telegraphy in North America, and Friedrich Gerke simplified Vail's code into the version adopted in Europe, which most of today's international Morse alphabet copies.

The telegraph made its public debut in 1844. On 1 May, news that the Whig Party had nominated Henry Clay for president was telegraphed from Baltimore to the Capitol in Washington. On 24 May, Morse officially opened the line by sending "What hath God wrought", a Bible verse chosen by Annie Ellsworth, whose father had championed the invention and secured early funding for it. The line could carry thirty characters a minute.

Morse code remained the international standard for maritime distress until 1999. Its best-known signal, SOS, is not sent as three letters but as a single prosign with no gaps, meaning "this is the start of a distress message". When the French Navy stopped using Morse on 31 January 1997, its final message was: "Calling all. This is our last call before our eternal silence."