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Signals

Most codes around you are trying to be read. The interesting question is why they were made at all.

 
Learn morse in half an hour

A lamp is a signal

Short flash, long flash, and a gap between letters. That is all morse is, and it works down a wire, across a valley, or through a wall.

Everyone in sight can read it. Signals carry a message but cannot keep it. For that you need a cipher.

Why does a code exist?

To hide
Ciphers, pigpen, and the rest of the ciphers page. The only group whose whole purpose is to stop people reading.
To save space
Resistor bands, barcodes, hexadecimal, airport codes, chess notation. Not secret at all, just too long to write out.
To cross a barrier
Braille through fingers, morse down a wire, semaphore across water, hazard symbols across languages.
To mark belonging
Uniforms, jargon, a catcher's signs, the words your family uses. Nobody is hiding anything; the code says who you are.
Not a message at all
The digits of pi run forever and say nothing. Knowing the difference is half of code-breaking.

Ways to send a message

There are three ways to encode anything. By resemblance, like the wiggle in a record groove. By position, like the pins on a music box. By agreement, where three dots mean S because we all said so. Only the third can keep a secret.

A record grooveThe wall of the groove wiggles exactly as the air wiggled. Play it with a needle and a paper cone A sundialThe shadow does not stand for the time. The shadow is the time A thermometerHotter air, taller mercury. Nothing to agree, nothing to learn A seismographThe pen shakes because the ground shook, and the paper keeps the shape of it A footprintThe oldest recording there is Music box pinsWhere the pin sits around the drum is when the note plays; which tooth it plucks is which note
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Codes that resemble

A record grooveThe wall of the groove wiggles exactly as the air wiggled. Play it with a needle and a paper cone A sundialThe shadow does not stand for the time. The shadow is the time A thermometerHotter air, taller mercury. Nothing to agree, nothing to learn A seismographThe pen shakes because the ground shook, and the paper keeps the shape of it A footprintThe oldest recording there is

Codes made of position

Music box pinsWhere the pin sits around the drum is when the note plays; which tooth it plucks is which note Piano rollsThe same idea in paper, with holes instead of pins Sheet musicHigher up the page is higher in pitch, further right is later in time Punched cardsA hole or no hole, in a place that means something The Jacquard loomPunched cards wove patterns into cloth, and gave Babbage the idea for a computer

Visual signals

FlagsOne flag for every letter, and each one also a whole sentence SemaphoreTwo arms, one position per letter, readable across a field Hand signalsHands can spell, and whole languages are built from them Smoke signalsCover the fire, lift the blanket, and the sky carries it Light and lamp signalsA shutter over a lamp turns light into letters Signal mirrorsSunlight aimed at a hilltop, visible for miles

Read by touch

BrailleSix dots read with a fingertip. Writing that needs no light at all Knotted cordA message you can wear on your wrist Tally marksThe oldest counting there is, still scratched on walls

Sound signals

The NATO alphabetAlfa, Bravo, Charlie: whole words so a bad radio cannot ruin them DrumsTalking drums copy the rise and fall of speech itself BellsCounted rings: watches, warnings, weddings, fires WhistlesWhistled languages carry across valleys where shouting fails HornsOne long note that means the same thing to everybody

Encoded signals

Morse codeDots and dashes, by wire, light, or tapping Tap codeTwo sets of taps for a row and a column, through a wall Knock codesAgreed knocks at a door: who you are before you speak Flash codesShort and long flashes, the same idea as morse with a torch

Modern signals

HexadecimalEvery letter is a number inside a computer, written in sixteens Resistor bandsColoured stripes on a part too small to print a number on Interface iconsPlay, save, wifi, bluetooth: a dozen codes you were never taught Radio signalsThe message goes to everyone, so the secret lives in the code Digital signalsOn and off, a million times a second QR and machine-readableA picture only a machine reads, printed on everything