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Imagine it's pre-industrial times, and a king wants to keep control of his vast empire. One of the biggest problems was how slow it was to transmit information; it could take weeks to be notified of invasion or rebellion. So, finding faster ways to communicate was one of the key problems of the time, and the pursuit of this goal led to semophores, radio, the telephone, and eventually the internet.

My question is this: Would pulling string have worked as an effect telegraph system?

You could have multiple 'stations' connected by a light string (that would be sealed in some tube to prevent outside interference). The string would wrap around a lever or some mechanical instrument. Then, one station would pull the string, which in turn would pull the lever on the next station. Different positions on the lever could represent different characters, sort of like morse code. The next station would recieve the value based on where the lever has been pulled to, then would repeat the same action on the next lever until the encoded message reaches its destination.

So the question is this: What would be some problems with this system? How far apart could the stations be before the string becomes too long and heavy to pull? Would it even be possible to pull a string kilometres long, even if it was ultra-light? How quick could messages be sent via this method?

Mauricio
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Reuben Matthews
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There are two big practical problems with using string for signalling - it stretches, and it breaks easily. You can replace the string with wires or metal bars - this was the system used for controlling railway signals until electric power was available. This makes the system more reliable - but also heavier. The maximum range for such mechanical linkages is about a kilometre.

For long distance signalling a visual system such as mechanical semaphore or signal maps is at the same time simpler, more reliable, and more flexible.

gandalf61
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The current set-up faces several issues:

  1. The enemy could damage our wire network or there might be some internal fault, similar to the scene at 2:50.

  2. Earth's curvature imposes limitations on the length of the set-up.

  3. A lengthy, thin wire requires significant strain for significant stress generation that could be sensed, so our communication would not be as fast as expected.

Considering historical alternatives, methods like sound or light, as seen in "Lords of the Rings" or employed by the Byzantine Empire with fire pillars, offer more efficient signaling options. In 1565 A.D., Mughal emperor Akbar utilized row of drummers with one drummer at few hundred metres for notifications at regular intervals during the queen's delivery. British Empire used light signals with help of mirrors as depicted in this video at 8:50.

To determine signal speed, imagining the string as a spring allows for calculating wave speed if we know $k$ of our spring. We can apply formula $v=\sqrt{kL/{\mu}}$ where L is length of spring and $\mu$ is linear mass density and $k$ is spring constant.

Spring constant of our wire could be obtained by formula: $k={\gamma}A/L$ here A is cross section of wire and $\gamma$ is Young's modulus of wire material.

Of course we are talking about longitudinal wave because you have already mentioned that our wire is in protective covering so transverse wave could not be generated.

Harjot Dhillon
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  • Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. – Community Jan 31 '24 at 15:07
  • @Community I never knew community bot could talk. Anyways, I have added more information – Harjot Dhillon Jan 31 '24 at 16:27
  • Why is the curvature of the earth a problem? The wire curves with the surface, it can wrap around the planet laying flat on the ground. I don't see why tension being non-perpendicular to gravity would be a problem in the first place, there is no issue running a wire up and down a mountain. – Nuclear Hoagie Jan 31 '24 at 18:22
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If the string was placed over the ground, friction would become a problem for large distances, when trying to communicate some movement.

On the other hand, for a string free in the air, it assumes the form of a catenary due to its own weight.

Any solution should seek for a material with low density to minimize that, and at the same time having high strength, to be possible to be stretched without break.

According to this article, silk has a remarkable strength, being at the same time pretty light. And it is a material well known since the antiquity, at least from Chinese emperors.

Doing a simulation based on the differential equations of the catenary, supposing to stretch it close to its strength limit, a free string is 4 m below the initial vertical position in the middle of 1km of distance. After 2 km, it is 16 m below.

I suppose that for that distances, visual signs along the great wall would be much cheaper and reliable.

Claudio Saspinski
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