How did Enigma operators transmit encrypted messages during World War II?

During World War II, German operators used the Enigma machine to encrypt messages through a process involving both electrical and mechanical components.1

To transmit a message, the process generally followed these steps:

• Setup: Operators received daily codebooks containing instructions for setting the machine's rotating wheels and plugboard to specific, predetermined positions.23
• Encryption: A cipher clerk typed the message in plain German on the machine's keyboard. As keys were pressed, electrical currents passed through a series of rotating wheels, which constantly changed the internal connections.1
• Output: Instead of printing the text, the machine illuminated letters on a lamp board. An assistant recorded these illuminated letters by hand to form the enciphered message.1
• Transmission: Once the message was fully enciphered, it was transmitted via Morse Code.1

To increase security, operators used a unique message key for each individual transmission, which was itself encrypted using the day's primary key. This complex system was designed to make the messages unbreakable, though it was eventually compromised by Allied code breakers at Bletchley Park through the Ultra intelligence project.241