Electric Telegraph

The electric telegraph sends messages along a wire as pulses of current from a battery, switched on and off with a key and read at the far end by an electromagnet. William Cooke and Charles Wheatstone patented a needle telegraph in Britain in 1837, Samuel Morse and Alfred Vail demonstrated their recording telegraph in 1838, and within thirty years cables linked continents.

What it is

An electric telegraph sends written messages as electrical signals along a wire. At one end an operator opens and closes a circuit; at the other end an instrument moves each time the current flows. Letters are spelt out with a code. In Cooke and Wheatstone’s first five-needle instruments, needles pointed to letters on a board; later needle instruments used a code of left and right swings.

The plate shows the simplest form, the single-wire line used with Morse code: a battery, a key, a line of poles and, at the receiving station, a sounder. The ground itself carries the current back.

How it works

The battery is a row of cells, each a pair of zinc and copper plates in a liquid, joined in series so their voltages add up. One end of the battery goes to the earth through a buried metal plate, the other to the key. The key is a sprung brass lever: pressing it closes a contact and connects the battery to the line.

A Morse key: pressing the knob closes the contact under the lever.

Current runs along the line wire, carried on glass insulators so it does not leak into the wet wood of the poles, and through the coils of an electromagnet at the far station. From there it goes down into the ground at a second earth plate and returns through the soil to the battery. Using the earth as the return path halves the amount of wire.

While current flows, the coils become a magnet and pull down an iron armature fixed to a pivoted lever, which strikes a stop with a click. When the key is released, a spring lifts the lever against a second stop with a different click. A short press gives a dot and a longer one a dash, and groups of them stand for letters and figures. Morse’s first instruments embossed or inked the dots and dashes on a moving paper strip, but operators soon found they could read the clicks by ear.

How to build it

A working key and sounder can be built on a table and run from a small battery, which makes the principle easy to see.

Wind the electromagnet. Wind about 30 metres of enamelled copper wire 0.2 mm in diameter in neat layers around a large iron nail or bolt about 10 cm long, leaving both ends free. Scrape the enamel off the last centimetre of each end. This coil has a resistance of about 16 ohms, so a 4.5-volt battery of three AA cells drives less than 0.3 amperes through it.

Make the sounder. Drill a hole in a wooden board and glue the nail into it upright, head up, without squeezing the winding. Cut a strip about 1 cm wide and 8 cm long from the tinplate of a food can, file its edges smooth, and screw one end to a block of wood so the free end sits a few millimetres above the head of the nail. Bend it so it springs back when pushed, and fix a screw in a small wooden bracket just above the free end, so the strip clicks against the nail when it is pulled down and against the screw when it springs back. Adjust the gap to 2 or 3 mm, bending the strip until it is pulled down firmly and still springs back.

Make the key. Screw a strip of springy brass or a folded piece of tin to another board so that one end rises a little. Put a drawing pin or screw under the free end as a contact.

Wire the circuit. Connect one coil end to one terminal of a holder with three AA cells. Connect the other battery terminal to the fixed contact of the key, and the key strip back to the other coil end. Use ordinary bell wire, up to about 10 metres, between the two boards: longer or thinner wire adds resistance and the strip may no longer pull down.

Send. Press the key: the strip snaps down onto the nail with a click and springs up when released. Agree on a code with a partner and send short words.

Safety. Use only a small battery of a few volts and never connect anything to mains electricity. Do not leave the key pressed for long, because the coil and the cells warm up, and disconnect the battery when you stop. Never join the battery terminals directly. Cut tin edges are sharp, so file them and wear gloves.

History and evolution

Before electricity, Chappe’s optical telegraph passed signals from tower to tower; a message could go from Paris to Lille in about half an hour.

The word telegraph first named the optical system of Claude Chappe and his brothers, demonstrated in 1791. Its first line, from Paris to Lille, opened in 1794 and covered about 230 km. Operators in towers moved wooden arms into set positions and read the next tower through a telescope. At its height the French network ran to nearly 5,000 km.

Electric telegraphs became practical once batteries could supply a steady current and electromagnets could detect it. In Britain, William Fothergill Cooke and Charles Wheatstone patented a needle telegraph in June 1837, described with five needles, and tried a needle instrument between Euston and Camden Town stations in London the same year. In 1839 the Great Western Railway installed one of their telegraphs on about 21 km of line from Paddington. Later British instruments needed only one or two needles.

In the United States, Samuel Morse, a painter and professor in New York, worked with Alfred Vail, who built the instruments. They demonstrated their telegraph at the Vail family’s iron works near Morristown on 6 January 1838, over about two miles of wire. In 1843 Congress paid $30,000 for a line from Washington to Baltimore, and on 24 May 1844 Morse sent the words “What hath God wrought” over it. By 1850 lines reached every important town east of the Mississippi, and one reached California in 1861.

The deep-sea Atlantic cable of 1865–66 in section: seven copper wires in gutta-percha, a layer of tarred jute, and ten iron wires each wrapped in hemp.

A submarine cable linked Dover and Calais in 1851. The first Atlantic cable, laid in August 1858, failed after a few weeks; a lasting one was landed in July 1866. Engineers learnt to send several messages over one wire, first one in each direction at the same time, with the duplex perfected by Joseph Stearns in 1868, then two in each direction with Thomas Edison’s quadruplex. British telegraph traffic peaked around 1900 at about 90 million messages a year. The telephone then took most of the work, and by 1980 satellites and new cables had made submarine telegraphy obsolete.