Morse code is an alphabetic code of long and short sounds, originally transmitted by telegraph. Each letter in the alphabet has a corresponding sound or series of sounds unique to it. The long sounds are referred to as dashes, while the short sounds are dots. Varying lengths of silence denote spaces between letters or words.
To make a dot on a telegraph, the telegraph key or switch was depressed and allowed to quickly spring back. To make a dash, the key was held down longer before allowing it to rebound. Messages were sent by tapping the key in a rhythm of coded letters. Messages were received via a radio transceiver, sounding like dots and dashes of static.
American Samuel Finely Breese Morse (1791-1872) invented the telegraph and this code in 1836. It was successfully tested on 24 May 1844, when Morse himself sent the first message between Washington DC and Baltimore: "What has God wrought?"
The most well known Morse code phrase is SOS (save our souls). SOS was chosen because the code for it — three dots followed by three dashes followed by three dots — is unmistakable as anything else and recognizable even to those who do not know the code.
Before SOS, the code was CQ which meant anyone listening, please respond. A third letter followed, revealing the reason for the hail. In the case of distress, it was a "D." When the Titanic hit an iceberg shortly before midnight on its maiden voyage in April 1912, operator John G. Phillips sent a mayday message using the old emergency code and the new one. Titanic's exact transmission that cold night was, CQD CQD SOS SOS CQD DE MGY MGY. "MGY" were Titanic's call letters, while "DE" meant from. The innocuous looking message literally translated to:
All hail our distress!
All hail our distress!
Save our souls!
Save our souls!
All hail our distress!
From Titanic!
The California was less than 20 miles (32 km) away and had enough boats to save everyone aboard Titanic, but their radio officer was off duty because it was the middle of the night. Titanic tried to get their attention by firing rockets. On duty officers aboard the California watched the rockets but failed to understand. The next morning when the ship's radio operator resumed duty, he learned from other ships what had happened. The Carpathia did respond to Titanic's distress call immediately, but that ship was 58 miles (93 km) away. By the time Carpathia arrived, it was too late for more than 1,500 of Titanic's passengers. Because of this disaster, it became law that a ship must always have a radio operator on duty.
Telegraph operators created shorthand that endures today in completely unrelated settings. One example is the use of "30" by reporters to mark the end of their copy. This was code for I have no more to send.
Morse code is still used today by the Maritime, Military and Amateur Radio Services. The code can also be sent by light, using short or long flashes to denote dots and dashes.