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Caesar wheel

Make a Caesar wheel from card and a split pin: two rings of letters that turn, and the whole cipher changes with them.

about 20 minutes needs scissors makes a working machine

What you need

The printed template
Card or thick paper
Scissors
A split pin, or a drawing pin and a rubber

How to build it

Print and stickPrint the template, glue it onto card, and let it dry flat.
Cut out both circlesThe big one and the small one. Take your time on the edges.
Pin them togetherThrough the middle, so the small circle spins freely inside the big one.
Choose a shiftTurn the inner circle until A on the outside sits above your chosen letter.
Read your message offFind each letter on the outer ring and write down what sits underneath it.
Tell your friend the shiftOne number, and they can turn their wheel to match.
Cut the small circle a little inside the line rather than outside it. If it is a hair too big it will jam.

The template

Print this at 100 per cent, not fit to page, or the sizes will be wrong. The ruler line below tells you whether it printed correctly.

10 cmABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZcut both circles out, pin through the centre
The bar above should measure exactly 10 centimetres. If it does not, your printer has shrunk the page.

Now send something

Set the wheel so A sits above D, then write THE DOG ATE MY HOMEWORK. That is a shift of three, which is exactly what Caesar used.

Where it comes from

Rome, and then everywhere
The cipher that would not die

Caesar shifted his letters three places. Two thousand years later the same idea turned up on cereal-box decoder rings, in comic-book fan clubs, and in the pocket of nearly every child who has ever invented a secret code. The wheel is what makes it quick: without one you count on your fingers and make mistakes.

Why it works

Every letter moves the same distance, so the pattern of the language survives. E is still the commonest letter, just wearing a different hat. That is why a shift cipher falls to counting, and why the wheel is a toy rather than a weapon.

When it goes wrong

It usually does, the first time. Almost always one of these.

The wheel jamsThe inner circle is a hair too big. Trim just inside the line, a little at a time.
It spins too freely and loses your placePush the pin through a small square of card behind it, or use a drawing pin into a rubber.
Your answers are one letter outSomebody read the wrong ring. Agree which ring is the message and which is the cipher before you start.

Where to go next

The shift cipherBack to Make