Light Telegraphs

Using just a single difference to communicate the alphabet became prevalent in the 17th century.

In combination with the telescope (invented 1608), the X device enabled utilizing this method across much greater distances. This meant a message could be sent from London to Paris, which instigated a surge in new inventions for passing differences.

The shutter telegraph was developed by Lord George Murray. Technology from 1795 demonstrates the use of a single difference to communicate anything.

Composed of six rotating shutters, in a two-by-three grid.

Each shutter could be either open or closed.

That is 2 to the power of 6, which means 64 differences in total.

Example:

a decision tree visualization of this creates a path like this:

all the letters of the alphabet and a set of numbers are at the end of the decision tree.

So to get to B, the sequence would be

closed closed closed closed open

Exercise