Today it is difficult to imagine a world without instant long-distance communication. Yet it was less than two centuries ago that the first telegraphic message became possible, and a few decades after that before the first practical telephone was invented. It must have seemed like magic, invisible signals traveling hundreds of miles through an apparently inert pair of copper wires.
Both the telegraph and the telephone rely on the same basic principle found in an ordinary ceiling light switch. Flipping the switch closes the circuit, allowing electrons to flow through the wire all the way to the lamp in the ceiling and back. The light bulb glows when current flows, and is dark when there is no current. By switching the current on and off, patterns of information can be transmitted from the switch to the light.
The Early Electric Telegraph
The first telegraphs did not use a light bulb, but rather a simple electromagnet that moved a lever with a stylus into contact with a moving strip of paper. The stylus would leave a long mark or a short mark, depending on how long the operator at the other end held down the telegraph key. Messages were sent using Samuel Morse's code, where each letter is represented by a particular combination of dots and dashes.
The electromagnet that drives the telegraph's stylus is no different in principle from the electromagnet found in an audio speaker. The only difference is that the audio speaker's magnet switches on and off much faster -- thousands of times a second -- causing the speaker membrane to vibrate, producing a sound. The newly-invented microphone replaced the telegraph key, encoding sounds picked up by the microphone into a pattern of electrical voltages which would reproduce the original sound on a speaker at the other end.
Similarities and Differences
Telegraphs generally required skilled operators who knew Morse code, and so most people never had telegraph machines in their homes; to send a telegram, one would go to a local telegraph office. The telephone, on the other hand, required no special skill; you only needed to speak into the microphone, and listen through the earpiece. In time, sophisticated switching networks were developed that allowed telephone users to dial each other directly. Similar technology was used for telegraphy (telex) until quite recently.It is interesting to note that the earlier invention, the telegraph, was actually a digital device in the sense that the data it transmitted consisted of simple on-off patterns, and its output was a string of fixed symbols. Now, of course, cellular telephones are used to send text messages as well as voice; telephones and telegraphs are again united.