Chappe optical telegraph

The Chappe telegraph is the first long-distance telecommunication system in history. Invented by Claude Chappe and his brothers Ignace and René during the French Revolution, it went into service on 15 August 1794 between Paris and Lille (230 km, 22 towers, 8 minutes per message). It predated the electric telegraph by 40 years and operated in France until 1855.

For 60 years, all important news in France travelled along the articulated arms of the Chappe telegraph: decrees, troop movements, financial markets, ministerial orders. The network would eventually cover 534 stations along 5000 km of lines.

Principle

The Chappe tower

Each tower carries on its top:

1. A large horizontal arm (the regulator), which can take 4 positions: horizontal, vertical and two diagonals (45°).
2. Two small articulated arms (indicators), one at each end of the regulator, each capable of 7 distinct positions (multiples of 45°).

Theoretical combinations: 4 × 7 × 7 = 196 distinct positions. In practice, 92 positions are meaningful (the rest serve as markers, control signals, end-of-message tokens).

The code

The Chappe code evolved over time:

• 1794 — 92-signal alphabet covering 26 letters + digits + punctuation.
• 1830 — extension to 8000 codewords (one signal = one word or whole phrase).

For proper nouns or non-standard words, letter-by-letter spelling uses the alphabetic subset.

Transmission

1. The operator at the starting tower sets the arms to the first signal’s position.
2. The operator at the next tower (5 to 15 km away) reads the position with a telescope and reproduces the same position on their own tower.
3. The signal propagates from tower to tower at 250 to 700 km/h depending on weather conditions.

A Paris–Toulon transmission (1000 km, 120 stations) took about 20 minutes for a short message.

History

The invention (1791–1794)

Claude Chappe, an engineer and ex-priest (the Revolution had abolished his vows), experimented with several systems:

• 1791 — first acoustic prototype (synchronised bells) — abandoned.
• 1792 — pivoting-panel system (black and white) — worked but hard to read.
• 1793 — articulated-arm system — the system that would prevail.

The National Convention, interested in military communications along contested borders (revolutionary wars in progress), commissioned the first line in August 1793.

The Paris–Lille line (1794)

15 August 1794 — first operational transmission, which immediately announced:

“Condé-sur-l’Escaut has been restored to the Republic; surrender took place on 30 August at six in the evening.”

This dispatch, transmitted in one hour, would have taken a day and a half by courier. The strategic implications were obvious: the Convention decided to extend the network to every frontier.

The network (1794–1855)

Expansion was rapid:

• 1798 — Paris–Strasbourg.
• 1803 — Paris–Brest, Paris–Milan.
• 1816 — full coverage to Atlantic and Mediterranean ports.
• 1830 — peak coverage: 534 stations, 5000 km of lines.

The Treasury invested heavily; the telegraph was a state monopoly reserved for government and military authorities. Private use was strictly forbidden (with the notable exception of stock-market quotes, whose fast transmission was worth a fortune).

The Blanc affair (1834)

In 1834, two banker brothers from Bordeaux, François and Joseph Blanc, bribed a Tours telegraph operator to insert, at the end of each official message, a specific control signal depending on whether the Paris stock exchange had risen or fallen. This signal, seen from Bordeaux 2 minutes later (instead of 2 days by courier), let them take financial positions ahead of the Bordeaux market.

For 2 years they made a fortune. The scam was uncovered by accident in 1836. With no law on telecommunication tampering (the concept didn’t exist), the trial ended in dismissal. The episode pushed lawmakers to pass, in 1837, the first law on the secrecy of communications — a direct ancestor of modern cybersecurity legislation.

The decline (1840–1855)

The arrival of the electric telegraph (Morse, 1844) tolled the bell for the Chappe system:

• Faster (instantaneous vs minutes).
• Insensitive to fog, rain and night (Chappe only worked in clear daylight).
• Cheaper to operate (1 operator per station vs 2 for Chappe).

The last Chappe lines closed in 1855; the towers were dismantled or repurposed (inns, scenic viewpoints, observatories).

Why it isn’t really a cipher

The Chappe code is a public transmission system, not a secrecy device. It carried state messages whose confidentiality rested on:

• The physical inaccessibility of operators (towers were army-guarded).
• Speed: between sending and arrival, the information had no time to leak through other channels.
• For genuinely secret messages (diplomacy, espionage), a classical cipher (Vigenère, nomenclator) was applied before Chappe transmission.

The 25 Chappe letters

One mast pose per letter. J is merged with I in the historical alphabet: any J in the cleartext is folded into I at encryption time.

A

B

C

D

E

F

G

H

I/J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

Digits and symbols

0

1

2

3

4

5

6

7

8

9

&