11 key questions about optical telegraphs
It’s a technology so outdated you’ve probably never heard of it. But optical telegraphs used to be the quickest way to get a message from point A to point B. Learn how—and why—they worked.
Q: What is an optical telegraph?
That’s a good question. They’re weird, but they used to be revolutionary. Here’s how they worked.
Someone was stationed at a tower with a signaling device on top. They put up a signal and waited. At a second station, usually miles away, a worker had a telescope trained on the first tower. When he saw the symbol, he recreated it on his tower. In this way, each symbol was passed down the chain from one end of the country to another.
Those symbols stood for letters, numbers, or other items in a code book. When they were put together, a message had successfully been passed from one end of the chain to another.
If you need an analogy, it was like a solid version of smoke signals (with a few more stops along the way).
Q: And people really did this?
Yes. It was the first telegraph and helped long messages travel quickly.
Q: Did it have other names?
The optical telegraph is also known as the semaphore telegraph, the visual telegraph, the shutter telegraph, the Chappe telegraph, or Napoleonic semaphore.
If you recognize semaphore, it’s because that’s the name we still use for flag signals.
How it worked
Q: Since it’s called optical semaphore, did they use flags?
No. The most popular system, developed by the French Chappe brothers, used black arms that contrasted against the sky. They could be arranged in different patterns.
The most popular alternative method was the shutter system. Shutters were shown or hidden in a pattern that corresponded to a code. Later, some people experimented with using lamps at night.
Q: How far apart were the stations?
It varied. On a clear day, you could see a station from ten miles away.
Q: So they transmitted letters of the alphabet?
Not necessarily. For example, the Chappe system usually transmitted positions in a code book. Many operators had a key with about 10,000 words in it. The telegraph would send a symbol for a page number and position, and when it got to the end of the line, the recipient would look up the words in their key.
This helped keep messages out of the hands of operators, but it probably also increased the number of errors in their messages.
Q: Who invented the optical telegraph?
There were experiments before, but credit for the invention of the telegraph belongs to Robert Hooke. In 1684, he proposed a system of symbols hidden by a screen. Hooke included theoretical distances between stations, ideas for how quickly they’d work, and more. Soon after, a French academic proposed a similar method. However, both ideas were merely theoretical. A few experiments followed (Sir Richard Lovell Edgeworth used a telegraph to spread news about a horse race), but the idea wasn’t regularly put into practice.
It wasn’t until 1794 that the Chappe brothers made the optical telegraph work. They developed a pivoted arm that could shift into different positions. In 1793, a line was built from Paris to the North. It took off from there.
Q: Was it popular? Why?
Yes, it became popular thanks to military interest.
In France, Napoleon was excited by the technology and built a line to Italy and another to Boulogne. More lines went to Marseille and Bayonne.
At its peak, there were 530 French towers, and one network extended from Venice to Amsterdam.
Q: So it was a French thing?
France was the biggest proponent of the optical telegraph, but it also made waves in Sweden and the UK. There were scattered appearances in Canada, the USA, and other countries.
Q: What happened to them? Why did people stop using them?
Basically, the electrical telegraph we know and love replaced them.
By the 1840s, telegraphs and Morse code were becoming established. Since electrical signals worked any time of day, were harder to intercept, could work in bad weather, and could go farther, the advantages were clear and the optical telegraph was on its way out.
Q: Was there any resistance?
Yes. It’s tough to nail down when people really stopped using the optical telegraph—reports vary from the 1860s to as late as the 1880s. It’s safe to say optical telegraphy was in use through the 1870s.
There were reasonable objections to ditching optical telegraphy for the electrical kind. As one commentator noted in 1841:
Every sensible person will agree that a single man in a single day could, without interference, cut all the electrical wires terminating in Paris; it is obvious that a single man could sever, in ten places, in the course of a day, the electrical wires of a particular line of communication, without being stopped or even recognized.
It’s not an unreasonable argument, but it went against the tide of history. It makes you wonder what technology we’re clinging to in the same logical, but ultimately backward, way.