The telegraph key is simply a switch in an electric circuit that turns on an electric current.
As the sender taps out a word the switch completes a circuit which allows an electric current to flow around it. As with all electric circuits, at least two wires are needed between the sender and the receiver -although for the very simplest of telegraph circuits, just one real wire and a return path through the earth is normally sufficient.
At the receiving end of the circuit the electric current can be used to provide power to an indicator dial or pointer machine (so the operator can watch the message coming in), to an electric 'sounder' or a buzzer (so the operator can hear the message) or to a device printing on paper tape (so the message can be stored and read later).
Magnetism begins within the atom itself and is caused by the movement of the atom's electrons.
There are many different sorts of magnets, but they all have the same properties and they all affect the world around them in the same way.
All magnets have north and south poles, and a magnetic field flows from one pole to the other.
A magnetic field exerts a powerful force on other magnetic materials - opposite poles of magnets are attracted to each other, and similar poles are repelled. A compass needle always points north, under the influence of the Earth's own magnetic field.
A 'permanent magnet' is in effect a large number of small magnets joined together - the magnetic fields of their atoms are lined up to create a strong magnetic field. 'Ferromagnets' (magnets made of iron or steel) don't start off magnetic, but once magnetised, remain that way. And 'electromagnets' only become magnetic only when an electric current flows in a coil surrounding them.
The needle telegraph moved fairly quickly from five needles to one and it was simple enough to understand a message with a little practice. One swing to the left could mean one letter, while two to the right might indicate another. The problem was trying to write it out at the same time as reading it.
The solution was to match the movement of the needle to sound. There were several attempts at workable machines, but this elegant piece of machinery, crafted by an engineer named Sir Charles Tilson Bright was one of the first to succeed.
He linked the movement of the needle to sounders on either side of the dials. If it flicked to the left it rang 'ting' and to the right 'tong'. This way the operator could listen to the message ringing out of the telegraph without needing to look at the dials and could concentrate on writing it down correctly.
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