Resource Cheatsheet
Code: Telegraphs and Relays
Code ch. 7 — the telegraph key and sounder, why long lines fail, and the relay: a switch flipped by electricity that regenerates the signal.
- Updated
- Reading time
- 2 min read
- Detail
- Code (2nd ed.)
- Detail
- Ch. 7
On this page
In one line: the relay — a switch operated by an electromagnet — lets one circuit control another, and that idea is the seed of every computer.
Companion to: Charles Petzold, Code (2nd edition), chapter 7.
The telegraph
| Part | How it works |
|---|---|
| Key | A spring-loaded switch the operator taps to send dots and dashes |
| Line | A long wire, using the earth as its return path |
| Sounder | An electromagnet that pulls an iron bar down with a click when current flows |
- Electromagnet: a coil of wire around iron. Current makes it magnetic; no current and it lets go.
- The problem: over hundreds of miles, wire resistance leaves too little current to work the sounder.
The relay
weak incoming line ──► [electromagnet] ──pulls──► [switch] ── fresh local battery ──► next line| Feature | Why it matters |
|---|---|
| Input current energises a coil | A tiny current can control a big one |
| The coil closes a separate switch | The output circuit has its own fresh power |
| Output copies the input | The signal is regenerated — relays chained along the route send it any distance |
| It’s a switch controlled by electricity | Relays can control other relays — the basis of logic gates (next chapter) |
Relay → vacuum tube → transistor: same job, each faster and smaller than the last.
🔐 Security and IT connections
- Regeneration is how networks span distance. Repeaters, switches and routers receive a weakened signal and send a clean new one.
- “Relay” still means “receive and pass on.” An open SMTP relay forwards anyone’s mail, so spammers abuse it. Mail servers should relay only for authenticated users.
- Relays still exist physically: PSUs, UPS units and industrial control systems (SCADA) use them to switch big loads from small signals.
Practice drills
1. What does an electromagnet do when current stops?
It loses its magnetism and releases the iron bar or switch contact.
2. Why did long telegraph lines need relays?
Wire resistance weakened the current. A relay used the weak signal to switch a fresh local battery onto the next stretch of line.
3. What's an open mail relay and why is it bad?
A mail server that forwards anyone’s email without authentication — spammers use it to send mail that looks like it comes from you.
Key takeaways
- The telegraph used a key (switch) and a sounder (electromagnet) over long wires.
- A relay uses one circuit’s current to flip a switch in another, regenerating the signal.
- Electrically controlled switches are the building block of logic — and of computers.