The optical telegraph

In 1792 Chappe demonstrated his telegraph for the Convention Nationale, the legislative body that governed France during the First Republic. In a matter of minutes it sent government orders to the furthest reaches of the land and ensured the coordination of troops by relaying coded optical signals between semaphores 25 kilometres apart. Each semaphore had a mast with a pivoting regulator and two moving arms. Each arm had copper blades that could take eight positions; seven were actually used, allowing a combination of 196 different signals corresponding to a specific vocabulary developed by Chappe. The first line, between Paris and Lille, began operating in 1794 and the network was completed in 1830. The arrival of electricity soon afterward signalled the end of Chappe's system, although Alphonse Foy's electric telegraph gave the code a reprieve for a few years.