The String and Other Telegraphs

Miscellaneous means of communication and warning by other means than electrical


If you wanted to make a private communication link, radio or a wire intercom would probably be your first idea, and you could immediately get the necessaries from your closest Radio Shack. There are other methods, some quite old, which do not involve electromagnetic waves or electric circuits for transmission, which will be examined here. Optical telegraphs, which receive sufficient attention elsewhere, are not included, and neither are means involving physical transport of a message.

Robert Hooke appears to be the inventor of the string telegraph. He described how to make and use one in 1667, and it survived at least as a toy until recently. The modern version requires two empty tin cans and a length of string or other filament. Simply make a small hole in the bottom of each can, pass the string through, and knot the end of the string so it cannot be pulled through the hole. If you and your partner hold the cans so the string is taut, then whoever speaks into one can is easily heard when a ear is held to the other. A bare iron wire probably will give much better results than cotton string. Experiments can be made to determine the effective range of the link. The wire can be supported on pegs, or hung by lengths of string. Again, experiment will show the best methods of support, and the way to turn corners if necessary. We note that the sound is transmitted as longitudinal vibrations in the wire, which must be kept taut. The theory of the string telegraph is explained in Theory of the String Telegraph.

Linemen installing telegraph lines used to communicate with other parties by knocking on the poles, before the circuits were in service. Hammering on railway rails also can be heard a considerable distance away. By putting one's ear to a rail, a train can be heard approaching well in advance (or so it is said). Knocking on water pipes would be similar. All of these means resemble the string telegraph, but do not transmit the voice. Shouting is a possibility, of course, but it has limited range and is not secure, besides being rude. Alexander the Great had a huge megaphone that he used to harangue his troops. One source says that American Indians used a string telegraph, but this was probably a mistake for drum signals. Signal fires were indeed used, but the reported practice of using a blanket to make smoke signals is probably easier imagined than done.

Smoke signals are an optical signal, and there are many other ways to use light for signalling. The heliograph is remarkably effective, where cloudless skies are the rule. The French army found it very effectual in Algeria, and developed a regular portable apparatus, with mirrors, shutter, and telescope. The British army also used it, notably on the North West Frontier in India. The United States army, as well as surveyors, made use of the heliograph in the western deserts. In the Second World War, downed pilots in life rafts could signal with a very simple heliograph consisting of a stout mirror with an unsilvered cross in its centre. For years, these were a common army surplus item. By night, a simple lantern and a screen are sufficient to send Morse Code signals. The top of the line is a powerful searchlight with a shutter like a Venetian blind in front of it. The heliograph and the searchlight also provide a very secure link, since the signals cannot be seen unless one is in the direct line. Both are excellent for military use, since wire lines do not have to be laid and maintained.

Using modern electronics, a light beam can be modulated to carry the voice, as well as interrupted to send pulse-coded information. The beam can be infrared, making it invisible to third parties. The common TV remote control is such a device, using pulse coding. The light can also be transmitted through optical fibre, employing the 1 mm fibre that is very easy to use and can be extended on routes impossible to aerial propagation, such as between rooms in a building. A laser beam can be reflected from a distant window, and what is being said in the room can be detected in the reflected light.

A surprisingly useful means of communication is the speaking tube. The tube, perhaps 3/4" in diameter, can be led on a tortuous route, such as from the bridge to the engine room of a ship. At each end, there should be a bell-shaped termination or cone, stopped with a plug that includes a whistle like those used on teakettles. To use the tube, simply withdraw the plug and blow into the tube, which sounds the whistle at the other end. When your party answers, a conversation can be carried on as if you were face-to-face. This extremely simple and rugged device never needs batteries replaced. The bridge telegraph also joined bridge and engine room. This was a set of two dials mechanically connected to show the same indications, and to ring a bell to call attention. It was used primarily to give instant commands concerning the engines, when the engines were controlled from the engine room, not from the bridge.

A common one-way communications link was the bell wire between a pull in a room or an entrance door, and a bell in the servant's station, for summoning help. This was later electrified, and is now universally found as the doorbell. Church bells are another one-way link for a specialized purpose. Let's now look at some more acoustic and optical one-way links, which have particular uses as warnings.

Along the shore at night, lighthouses can be seen sweeping their beams over the sea. Here and there we see the lights periodically flashing codes that permits each light to be recognized and distinguished from the others, telling us where we are, and warning us to steer clear of dangerous rocks and shoals. Now the fog descends, and the lights disappear. We hear the groans of the foghorns, coded like the lights. We cannot take bearings on the sounds, but know when we hear them that we are near their source, and the loudness gives us some idea of how far away they are. Unfortunately, the audibility of foghorns is subject to great variations, but they are better than nothing, and tell us to beware. A typical foghorn is the Diaphone, which requires only a source of compressed air. It has a piston working in a cylinder, where there are matching holes that are covered and uncovered as the piston moves back and forth. These holes connect with an annular air chamber, so that puffs of air exit and the sound passes out a suitable horn. The air pressure also drives the piston back and forth automatically. A frequency of 180 Hz (about F below middle C) is typical.

The siren, invented by Seebeck (1805-1849), the discoverer of thermoelectricity, is a revolving perforated disk in a jet of air. Its scientific use was to make sound of a known frequency, but its practical use relies on its loudness. Puffs of air come out at a frequency determined by the speed of rotation and the spacing of the holes. When used for public warnings, it is best placed as high as possible for the greatest range. When used as an air raid or tornado warning siren, a constant pitch is used for testing and all clear. The danger signal is a rising and falling pitch, obtained by turning the motor that rotates the siren disc on and off. Unfortunately, the general public is not aware of this, and the officials may also be ignorant of it as well. A siren can include a centrifugal air pump so that it is independent of a source of compressed air, and most modern sirens are of this type. On emergency vehicles, it has now been superseded by electronic sirens that can make a variety of unpleasant noises.

Sirens and foghorns are acoustical beacons, used for warnings like the beacon fires that have been employed since time immemorial. Curiously, the word beacon is now often misused to imply a guiding light of some kind, when really it means a dire warning.


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Composed by J. B. Calvert
Created 7 May 2000
Last revised 7 May 2000