(Mirror of a Hacker News comment I wrote, responding to someone who wondered what a serial connection actually looked like “on the wire.”)
I binged CuriousMarc’s YouTube series on teletype restoration a few months ago, so here’s a simplified (and possibly somewhat inaccurate) explanation of how one works.
The only electrical components in a Teletype are (a) a continuously-running electric motor, and (b) an electromagnet and a few switches. Everything else is completely mechanical.
The Teletypes in a circuit, and the electromagnet and switches within them, are all connected in series, forming a current loop. (Current, rather than voltage, is used because you can use a constant-current power supply to get the same power at each electromagnet regardless of how many are in the circuit and how many miles of wire are between them.)
The Syko machine was a British strip cipher used during WWII for encoding radio communications between aircraft. There's not much information on the Web about these machines; a scan of the Syko Manual (PDF, 2.4MB) is available but it's rather hard to read as it's an old document and has had extra annotations added.
In this post, I have transcribed the entire document to make it easier to read and find information about how the Syko machines were operated.