Wig-Wag

Albert Myer worked out a signalling system for a single flag and got it adopted in 1860; by 1861 both sides were using it. Semaphore needs two flags and a clear view of both; wig-wag needs one flag, or a kerosene torch at night, and a hilltop.

The flag starts upright. A sweep to the sender’s left and back is a 1; a sweep to the right and back is a 2. A dip to the front is a 3, and marks the end of something: one for the end of a word, two for the end of a sentence, three for the end of the message.

Each letter is a short run of 1s and 2s - I is a single 1, T is a single 2, A is 22. Those codes are not prefix free, so the pause between letters is doing real work; without it, 1 2 and 12 are both readable and mean different things. Here that pause is a space.

Each numeral is five motions, which keeps them clear of every letter, and each also doubles as an instruction to the other station: 1 means wait, 6 means send faster, 7 means did you understand?

The codes below are the General Service Code of July 1864. Myer’s original had the 1 and the 2 the other way round.

Examples:

Wig-wag was read as easily by the enemy as by the station it was meant for, which is why a message sent this way was usually enciphered first.