Visual Telegraphs (case study) The history of visual telegraphs
Visual Telegraphs (case study)
⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.
- The signal fire is no doubt one of the oldest technologies for transmitting information.
- Perhaps dating back to the first controlled use of fire.
- It allows one person to influence another's believe state
- across a distance.
- Because with the ability to notice
- either the presence or absence of something
- We are able to switch between one of two believe states.
- One difference-two states
- If we look back in history,
- We find that this was of great importance to military powers
- which all rely on effective communications
- A great place to begin is with the Greek myth of Kadmos,
- a Phoenician prince who introduced phonetic letters to Greece.
- The Greek alphabet, borrowed from the Phoenician letters,
- along with a light and cheap papyrus
- effected the transfer of power
- from the priestly to the military class.
- Greek military history provides clear evidence
- of the first advancements in communication
- stemming from the use of signal torches.
- Polybius was a Greek historian born in 200 BC.
- He wrote "The Histories", which is a treasure trove of detail
- related to the communication technologies of the time.
- He writes: "The power of acting at the right time..."
- "...contributes very much to the success of enterprises..."
- "...and fire signals are the most efficient of all devices..."
- "...which aid us to do this."
- However, the limitation of a signal fire was clear to him.
- He writes: "It was possible for those who had agreed on this..."
- "...to convey information that, say, a fleet had arrived..."
- "...but when it came to some citizens having been guilty of treachery..."
- "...or a massacre having taken place in town, ..."
- "...things that often happened but cannot all be foreseen, ..."
- "...all such matters defied communication by fire signal."
- A fire signal is great when
- the space of possible messages is small.
- Such as "enemy has arrived" or "not arrived".
- However, when the message space
- - which is the total number of possible messages -
- grows, there was a need to communicate many differences
- and in the Histories Polybius describes the technology
- developed by Aeneas Tacticus
- one of the earliest Greek writers on the art of war,
- from the 4th century BC.
- And his technology was described as follows:
- "Those who are about to communicate urgent news to each other by fire signal..."
- "...should procure two vessels of exactly the same width and depth."...
- "And through the middle should pass a rod..."
- "...graduaded into equal sections..."
- "...each clearly marked off from the next..."
- "...and denoted with a Greek letter."
- Each letter would correspond to a single message in a lookup table
- which contained the most common events that occur in war.
- To communicate they would proceed as follows:
- First the sender would raise his torch
- to signal that he had a message.
- The receiver would then raise his torch,
- signalling he was ready to receive it.
- Then the sender would lower his torch
- and they would both begin to drain their vessels
- from a bored hole of equal size at the bottom.
- Now, when the event is reached
- the sender raises his torch
- to signal that they should both stop the flow of water.
- This results in equal water levels, denoting a single shared message.
- This ingenious method used differences in time to signal messages.
- However, its expressive capabilitiy was limited,
- mainly due to its speed.
- Polybius then writes of a newer method
- originally devised by Democritus
- which he claims "was perfected by myself..."
- "...and quite definitely capable of despatching with accuracy..."
- "...every kind of urgent message."
- His method, now known as the Polybius Square, works as follows:
- Two people, seperated by a distance,
- each have 10 torches, separated into two groups of five.
- To begin, the sender raises a torch and waits for the receiver to respond.
- Then the sender lights a certain number from each group of torches
- and raises them.
- The receiver then counts the number of torches lit in the first group.
- This number defines the row position
- in an alphabetic grid they share.
- And the second group of torches signifies the column position in this grid.
- The intersection of the row and column number
- defines the letter sent.
- Realise that this method can be thought of
- as the exchange of two symbols.
- Each group of five torches is a symbol
- which is limited to five differences:
- from 1 to 5 torches.
- Together these two symbols multiply
- to give 5 x 5 = 25 differences,
- and not 5 + 5.
- This multiplication demonstrates
- an important Combinatorial understanding in our story.
- It was explained clearly in the 6th century BC
- in the Indian medical text attributed to Sushruta,
- an ancient Indian sage, as follows:
- "Given 6 different spices, ..."
- "...how many possible different tastes can you make?"
- Well, the process of making a mixture
- can be broken down into in six questions:
- Do you add "A", yes or no?
- Do you add "B"?
- and "F"?
- Realise this multiplies into a tree of possible answer sequences
- 2 x 2 x 2 x 2 x 2 x 2 = 64
- 64 different sequences of answers are therefore possible.
- Realise that given n yes-or-no questions
- there are 2^n possible answer sequences.
- Now in 1605 Francis Bacon clearly explained
- how this idea could allow one to send all letters of the alphabet
- using only a single difference.
- With his bilateral cypher, Bacon wrote famously:
- "The transposition of two letters by five placings..."
- "...will be sufficient for 32 differences."
- "For by this art a way is opened..."
- "...whereby a man may express and signify..."
- "...the intentions of his mind..."
- "...at any distance of place..."
- "...with objects which are capable of a two-fold difference only."
- This simple idea of using a single difference
- to communicate the alphabet
- really took flight in the 17th century
- due to the invention of the telescope
- by Lippershey in 1608 and Galilei in 1609.
- Because quickly the maginification power of the human eye
- jumped from 3, to 8, to 33 times and beyond.
- So the observation of a single difference
- could be made at a much greater distance.
- Robert Hooke, an English polymath
- interested in improving the capability of human vision using lenses
- ignited progress when he told the Royal Society in 1684
- that suddenly, "with a little practice..."
- "...the same character may be seen at Paris..."
- "...within a minute after it hath been exposed at London."
- This was followed by a flood of inventions
- to pass differences more effectively
- across greater distances.
- One technology from 1795 perfectly demonstrates
- the use of a single difference to communicate all things.
- Lord George Murray's shutter telegraph
- was Britain's reaction to the Bonapartese threat to England.
- Is was composed of six rotating shutters
- which could be oriented as either "opened" or "closed".
- Here, each shutter can be thought of as a single difference.
- With 6 shutters, we have 6 questions: open or closed,
- providing us with 2 ^6, or 64, differences,
- enough for all letters, digits, and more.
- Now realise that each observation of the shutter telegraph
- can also be thought of as the observation of one of 64 different paths
- through a decision tree.
- And with a telescope it was now possible to send letters
- at an incredible distance between beacons.
- However, an observation in 1820 led to a revolutionary technology
- which forever changed how far these differences could travel
- between signaling beacons.
- This ushered in new ideas
- which launched us into the information age.
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site