You have to tap a line to intercept a telegraph message and most telegraph stations used were well behind the lines, and cut on the Rebel side to prevent tapping. Any tapping on your side of the line has to cross your lines to each the enemy.
That wheel shows the 'wigwag code' for signalling with flags.
en.wikipedia.org
Very interesting.
The secret code for telegraph became the Morse Code.
Morse code, invented 30 years before the Civil War, is a binary system of X & O was the basis of telegraphic communication… & how this iPhone I am using works as well.
The single flag / torch signal system used by both sides was invented by a military surgeon during his medical training. It was a very practical signing technique for deaf persons.
The beauty of Albert Myer's system was its simplicity. A great granddaughter can easily teach it to first grade boys.
The flag / torch system was as diagrammed above. The vertical "3" movement was for spacing / punctuation.
Myer's system was codified into the "General Services Code" used by the U.S. Military until 1913. It was this system, not Morse that was used for military communications.
Messages were written using abbreviations that any texting child will recognize. The official dictionary of abbreviations is lying next to the telegraph key.
The code system used on the U.S. Military Telegraph System was the granddaddy of modern coding. It was both simple to encode & to break. I can testify that small children delight in using it. It was, at that time, unbreakable without the key. No less a personage than Porter Alexander attested to that.
An intercept party disguised as wood choppers intercepted all of the telegraphic messages between Grant's HQ at City Point & Washington. They were delivered to Signal Service HQ in Richmond. They even advertised in the paper hoping to find some way to break them. They never did.
This is an authentic CSA code wheel. It produces the kind of substitution code that you may have used in grade school. Compare that to a Union wheel.
This wheel turns the alphabet into a meaningless binary symbol that was changed throughout the day.
During the Atlanta Campaign Capt Cole & Lt Floot of the Signal corps were a code breaking duo. The A of TN signalists habitually did maintain rigorous signal security.
From their station of observation, Cole & Floot captured "contrabands." The intercepts were cracked & Sherman read them while eating supper at his HQ at Big Shanty, e.g.
When the A of TN codes were changed, the contrabands were still collected. Cole & Floot would take only a few hours to crack the new code which would then be distributed through out the Union network.
It was a contraband saying, "Why has the wagon for Gen Polk not arrived?" that told Sherman that Gen Polk had been killed.
Obviously, this is hardly the venue for explaining CW codes / cyphers (synonyms at that time) & how they were used.
Warning: The CW cyphers led to Morse code quote above is from an incorrect Wikipedia entry that has been repeated many times. Nothing could be further from the truth.
Note: For excellent, detailed information the Civil War Signals webpage is unbeatable. One of the contributors, a former army signalist, was the mentor who helped begin the Signal Corps program at Stones River.
The most original of original sources is the Thomas T. Eckert collection at the Huntington Library in California. Major Eckert commanded the Washington military telegraph office during the war. Until 2009, it was believed that, following official policy, the logs had been destroyed. After the Huntington gained possession, there was a crowd funded digital conversion of the entire collection.
Online you can read Dana's personal log that was eyes only for Stanton & Lincoln. You can ignore the endless speculative twaddle about what Grant was saying by reading his raw messages. You can also read what was being reported to Grant.
The Ekert collection also includes the code books used to encode & crack messages.