The electric telegraph was the first high-speed long-distance communication method, using Morse Code to encode letters and numbers as pulses of electrical current. This video contains a quick explanation:
This video shows someone building a simple telegraph machine:
Then two different technological paths were taken. In one, Morse Code continued to be used but it was now broadcast by radio instead of being sent from point to point as electrical current in wires. These US Navy training videos from World War II demonstrate its use:
At the same time, the telegraph system began to use more sophisticated machines to speed up and improve the transmission of messages by wire. At first, they still used Morse Code, although they encoded the dots and dashes slightly differently, as explained in this video, which also shows the first use of punched holes in paper tape to store messages:
Later, Émile Baudot invented a new system to transmit messages faster. One of his innovations was a new encoding: unlike Morse Code, which is a variable-length encoding, Baudot used a fixed length for every character.
In Morse Code, different characters are encoded using different numbers of dots and dashes. For example, an E is a single dot, whereas an S is three dots. Morse Code also uses pauses of different lengths to encode the breaks between characters and the spaces between words, as shown in the US Navy training video above.
In contrast, Baudot's code, which was later modified by Douglas Murray to become the Baudot-Murray code, encodes all characters using symbols of the same length. It also includes symbols for punctuation, so spaces (and commas, etc.) are just characters like letters and numbers.
Every character is made up of five parts, in each of which the electrical voltage may be high or low. This is similar to how the later telegraph system encoded the dots and dashes of Morse Code, as shown in the Porthcurno Telegraph Museum video above, except that in the Baudot-Murray code every character is encoded as five dots or dashes.
In modern terminology, this is a 5-bit code. A bit can be in one of two states: either 0 or 1; either high or low voltage; either dot or dash. Just as in the Porthcurno Telegraph Museum video, these two states can also be represented by punching holes in paper tape. This video explains how messages using the Baudot-Murray code were stored on paper tape:
The big advantage of this system is the speed at which messages can be reliably encoded and decoded by machines. Teletypes are typewriters that can be remotely controlled: they receive messages using Baudot-Murray code and type them directly onto a page. Similarly, when characters are typed using the keyboard, they encode them using Baudot-Murray code and transmit them. This video shows a restored teletype sending and receiving Baudot-Murray encoded messages:
During World War II, the governments and militaries of the countries at war used radios and telegraphs to communicate. The US Navy training video above shows how radio messages using Morse Code were vital for co-ordinating military forces. However, they did not want enemy forces to be able to listen to their radio broadcasts and decode their messages. So they used encryption to disguise their messages before transmitting them. The German military used an encryption machine known as Enigma.
This video provides a brilliant explanation of how Enigma machines worked:
This video shows someone using a real Enigma machine:
These two videos provide another explanation of how Enigma machines worked, with an explanation of why their encryption was hard to break and the flaw in their design which eventually allowed the cipher to be broken:
These two videos provide another explanation of Enigma machines with much more historical context, followed by the effort at Bletchley Park to break their cipher:
Finally, this website provides a lot of technical detail on both how the Enigma machine was actually used, e.g. how they were configured for each message and how those message settings were transmitted, and how the Bombe actually worked:
The German high command communicated using a different encryption technique, the Lorenz cipher, which operated on messages encoded in 5-bit Baudot-Murray code rather than Morse Code.
This video gives an introduction to the Lorenz cipher, known to Bletchley Park as "Tunny":
And this video describes the technique that was used to recover a key from two ciphertexts encrypted using the same key, a "depth":
Having obtained a key, Bletchley Park managed to determine how the Lorenz cipher machine worked, without ever seeing the machine itself. That process is described in this video:
Bletchley Park then developed a statistical technique that allowed them to determine the Lorenz machine settings used for a particular message by analysing only the ciphertext. It was this technique that required the invention of an electronic digital computer, which they called Colossus. This video explains how that technique worked:
This video provides a bit more detail on Colossus itself, including why it was necessary:
This video shows a replica of Colossus running at The National Museum of Computing, at Bletchley Park:
This video from the Open University shows a bit more of the same Colossus replica:
Finally, this is a video of a talk given at the "ENIGMA – Precursor of the Digital Civilization" conference in 2015 in Warsaw, Poland. It has a few more historical details and more about Colossus itself: