The telegraph had gradually come into use in the two decades before the civil war. The North being far more industrialized than the South and accordingly had an more extensive telegraph network than the South. Consequently, the South had simple needs and used a variety of substitution ciphers of the type we are all familiar with. The North is another matter, it used what is called a route cipher and it stayed unbroken through out the war. That isn’t say that code books and operators weren’t captured through out the war but this necessitated changing the code books not changing the cipher method. There really isn’t a single Northern code as such, the code started out simple and as the war progressed the code was revised from time as code books or operators were captured or otherwise compromised. Additionally, there wasn’t just one code in use at any one time, there could be any number of codes in use as each telegraph pair could decide on which code they might use on any day. Additionally, the use of paper and pencil decoding methods made decoding too time consuming to be of any practical use.
Routing Cipher
Suppose you wanted to hide a geocache using Civil war techniques. You might start out buy writing out the message in plain text. “from a nearby location set your locator to north seventy two degrees twelve minutes point eight five three period then set the west coordinate to zero eight two degrees nine seven minutes point one one seven period the cache is down the path a short distance from the large stone period”