Morse code

The 1844 message

Samuel Morse sent that line from the US Capitol to Alfred Vail in Baltimore on 24 May 1844, opening the line Congress had funded. Annie Ellsworth, the patent commissioner’s daughter, picked the phrase out of the Book of Numbers.

Where the code lengths came from

Vail sized each letter by how often English uses it, counting the movable type in a Morristown newspaper’s cases: twelve thousand E, nine thousand T, two hundred Z. E became a single dot and T a single dash. Shannon and Huffman formalized the idea a century later. Vail got there with a type case.

What this table holds

Twenty-six letters, ten digits and eighteen punctuation marks of International Morse, the code Friedrich Gerke rewrote in 1848 and the International Telegraph Congress adopted at Paris in 1865. Morse’s American original used longer dashes and gaps inside several characters, so the output above is a later code than the message it carries. The newest entry is @, which the ITU added on 24 May 2004, a hundred and sixty years to the day after that message.

A space divides letters and a forward slash divides words. Case is dropped, and a character with no code is discarded rather than passed through, so an accented letter goes missing silently.

Prosigns

A prosign is two letters run together with no gap, making one symbol. Two common ones are reachable here through their punctuation twins: BT, the paragraph break, is the same signal as =, and AR, end of message, is the same as +.

Type SOS and you get ... --- ..., three letters with the gaps intact, where the 1906 Berlin convention specified one unbroken nine-element signal chosen to be unmistakable in noise. That distinction lives in the timing, and a text encoder has nowhere to put it.

Hearing it

Under whichever box holds Morse sit Play and Download .wav, with sliders for Speed, Characters and Tone. All three move a message that is already playing rather than stopping it, so the pitch can be found by ear and the speed pushed up once the letters start landing. Play SOS and you can hear the three gaps the 1906 signal did not have.

Speed is measured as it always has been. PARIS and the space after it comes to exactly fifty dot lengths, so one dot is 1200 divided by the words per minute, in milliseconds.

Setting Characters above Speed gives Farnsworth timing: each letter is keyed at the faster rate and only the gaps stretch. A learner at five words a minute with characters at eighteen hears the shape a letter will really have, with room to name it before the next arrives. Letters learned slowed down teach a rhythm that has to be unlearned.

Why it outlives its replacements

Morse needs a carrier that can be switched on and off: a key, a lamp, a mirror, a finger on a wall. Filmed for a propaganda interview in Hanoi in May 1966, Jeremiah Denton blinked T-O-R-T-U-R-E at the camera, and American intelligence read it off the broadcast.

On radio the gain is bandwidth. A keyed carrier occupies about a hundred hertz where speech needs three thousand, so it stays readable at signal levels where voice has already collapsed. That kept it on the maritime distress watch until GMDSS took over on 1 February 1999.

Related

Tap code strips it to one sound and a grid, which is how the American prisoners in Hanoi talked through the walls. The spelling alphabet covers the case where a voice channel exists.