00 What is the Chappe Telegraph?

At the time of the French Revolution, the French technician Claude Chappe succeeded in developing a technically practical optical telegraphy device based on the transmission of characters by means of pivoting signal arms (also known as a wing telegraph or semaphore).

Two pivoting crossbars were attached to a high mast, with two further pivoting bars at each end, by means of which different letters could be signaled on the basis of a code, depending on their position.

Several series of tests showed that the system was easy to operate and robust.

Thus, in 1794, the first regular telegraph line was established between Paris and Lille, covering 270 km with 22 semaphore stations.

The running time for the transmission of a single letter was an impressive two minutes at the time. The flexibility and speed convinced the military in particular to quickly set up a nationwide optical-mechanical telegraph network.

For message transmission, the crossbar had to be horizontal, vertical or diagonal.

The signal arms could each project at an angle of 45°, 90°, 135°, 225°, 270° and 315° or be folded back onto the crossbar.

This resulted in a total of 7 – 7 – 4 = 196 signals.

However, to avoid errors, only 98 were used in practice, 6 of which are reserved for signal preparation.

The vocabulary is represented as a directory/dictionary with 92 pages, each containing 92 words (or groups of words). To encode a word, therefore, two numbers had to be sent, the first being the page number and the second the line number (or word number).
Thus 92 – 92 = 8464 code words were available.

The word homme (man in French) was coded 43-51 according to the Chappe vocabulary used in Savoy in 1809.

Another codebook was digitized by Thérèse Eveilleau.