How the electric telegraph worked: keys, sounders and relays
The electric telegraph worked by switching a current on and off. A battery pushed current along a wire to an electromagnet at the far end, and a key at the sending end opened and closed the circuit. While the key was down, the magnet pulled an iron lever toward it; when the key came up, a spring pulled the lever back. A short closing made a dot, a longer one a dash, and the operator at the other end read the pattern.
Everything else in a telegraph office, the sounder, the relay, the register and the batteries, was there to make that one idea work over long distances.
The circuit: battery, line, key and magnet
A telegraph line is a loop that current can flow around, with a switch in it. These are the parts:
- The battery. Early telegraph offices ran on wet cells: jars of liquid with metal plates in them, joined in series to give enough push for a long line.
- The line. A single wire strung on poles, held off the wood by glass or ceramic insulators.
- The return path. A circuit needs a way back. Instead of stringing a second wire, most lines used the earth: each end was connected to a metal plate or pipe buried in damp ground, and the current found its way back through the ground. One wire on the poles was enough.
- The key. A lever on a pivot, with a pair of contacts under the lever and a spring that holds them apart. Press the knob and the contacts touch, closing the circuit. Let go and the spring opens it.
- The electromagnet. A coil of insulated wire around an iron core. With current in the coil, the core becomes a magnet; with none, it is plain iron. Just above it sits the armature, a piece of iron on a lever, held up by a spring.
Put them together and the whole system has two states. Key down: current flows around the loop and the magnet pulls the armature down. Key up: the current stops and the spring lifts the armature. The far end knows nothing except “on”, “off” and how long each lasted. Morse code is simply a pattern of those on and off times, which is why it survives so well on anything that can switch: a lamp, a buzzer, a radio transmitter.
The sounder: reading Morse as clicks
The first receivers were registers, which marked the dots and dashes on a moving strip of paper tape for the operator to read afterwards. Morse code on paper tape: how telegraph registers printed it looks at those in detail. Operators soon found that they could read the clicks of the instrument by ear, faster than they could read the tape, and on most lines the sounder replaced the register.
A sounder is an electromagnet and an armature lever built to be heard. The lever travels between two stops. When current starts, the magnet pulls it down onto the lower stop with a sharp click. When current stops, its spring throws it back against the upper stop with a second click that sounds a little different. The operator listens to the time between the two: a short interval is a dot, a longer one a dash, and the quiet before the next down click is the gap.
There is no tone in any of this. A landline telegrapher heard Morse as a rhythm of clicks and wrote the message down as it came in. The steady tone that most people now think of as Morse belongs to radio.
Relays: carrying the signal farther
Distance was the weak point. A long wire has resistance, and every insulator leaks a little current to its pole, more in wet weather. By the time the current reached a distant office, it could be too weak to snap a sounder’s lever down hard enough to hear.
The relay solved it by not asking the weak current to do the heavy work. A relay is an electromagnet built to be sensitive: many turns of fine wire, a light armature and a gentle spring, so that a feeble current can still move it. Its armature carries a contact of its own, and that contact switches a second, separate circuit inside the receiving office, made of a local battery and the sounder.
So the line current moves the relay, the relay switches the local battery on and off, and the local battery drives the sounder loud and clear. The sounder copies every movement of the relay, so the operator hears the dots and dashes exactly as they were sent.
| Part | Where it sits | What it does |
|---|---|---|
| Key | Sending office | Opens and closes the main line |
| Main battery | On the line | Pushes current along the wire |
| Line and earth return | Between offices | Carry the current out and back |
| Relay | Each receiving office | Moved by the weak line current; switches the local circuit |
| Local battery | Each receiving office | Powers the sounder |
| Sounder | Each receiving office | Turns the signal into clicks the operator can read |
On very long routes the same idea joined one section of line to the next. At a repeater, the relay of one section keyed the following section with its own battery, so the signal set off again at full strength. Where there was no repeater, an operator copied the message and sent it on by hand.
The idea outlived the telegraph. Relays of the same basic kind, a magnet closing a contact, were later used to switch telephone calls and to run some of the first computers.
Many offices on one wire
A railroad line often had many stations on the same wire, each with its own relay and sounder in the circuit. Every sounder on the line clicked with every message, so each office listened for its own call letters and let the rest go by.
On many American lines the circuit was kept closed when nobody was sending, with current flowing all the time. Each key had a small switch on its side, the circuit closer, that bridged the contacts while the key sat idle. To send, an operator opened the circuit closer, which opened the line, and began keying; when they had finished, they closed it again. An operator who forgot left the line open, and nobody else on the wire could send until it was closed.
The key itself has barely changed since. A straight key on an amateur radio desk today is the same lever, contacts and spring, keying a transmitter instead of a wire; How to adjust a straight key: spring tension and contact gap covers setting one up.
Try the idea on a table
You can build the whole principle in an afternoon, with nothing more dangerous than a couple of AA batteries.
- Make a line. Take two long pieces of insulated wire. At one end, put a push button (your key). At the other, put a buzzer or a bulb rated for the battery’s voltage (your receiver). Connect the battery so that pressing the button completes the loop.
- Send something. Stretch the wires across a room and have one person press the button while the other listens or watches. Send E and T first: one short press, then one long one. Then try a word, keeping the long presses about three times as long as the short ones.
- Make a magnet. Wind insulated wire many times around an iron nail and connect it to the battery through the button. While the button is down, the nail will pick up a paper clip; when it comes up, the clip drops. That is the heart of a sounder and a relay. Press it only briefly: the coil draws a lot of current, warms up and drains the battery quickly.
Use only ordinary low-voltage batteries for this, never anything that plugs into the wall.
If you would rather hear the code than wire it, Dot & Dash draws a telegraph-style machine on your Mac: a key with a knurled knob, a lever that moves as the signal goes out, and a paper tape that prints each message in dots and dashes. It plays a tone, as radio does, rather than clicking like a sounder.
Questions
Did telegraph operators read Morse from paper or by ear?
Both, at different times. The first receivers, registers, marked the code on paper tape, but operators soon learned to read the clicks by ear, and on most lines the sounder replaced the register.
What did a relay do on a telegraph line?
It let a weak current control a strong one. The current arriving over a long line was often too feeble to work a sounder, so it moved a sensitive relay instead, and the relay switched a local battery that drove the sounder.
Why did a telegraph line need only one wire?
Most lines used the earth as the return path. Each end was connected to a plate or pipe buried in the ground, so the current went out along the wire and came back through the earth.
Did the telegraph beep?
No. A landline sounder made two clicks for every dot or dash, one as the current started and one as it stopped, and operators read the time between them. The beeping tone people associate with Morse came with radio.