Signal lamps were pioneered by the Royal Navy in the late 19th century and continue to be used to the present day on naval vessels. They provided handy, relatively secure communications, which were especially useful during periods of radio silence, such as for convoys operating during the Battle of the Atlantic. Flashing lights were the second generation of signaling in the Royal Navy, after the flag signals most famously used to spread Nelson’s rallying-cry before the Battle of Trafalgar.
There were several types. Some signal lamps were mounted on the mastheads of ships, some small hand-held versions are also used (e.g., the ‘Aldis lamp’); other more powerful versions were mounted on. These larger ones used a carbon arc lamp as their light source and could be used to signal to the horizon, even in conditions of bright sunlight. Although it was originally thought that it was only possible to communicate by line-of-sight, in practice it is possible to illuminate cloud bases both during the night and day, which allowed for communication beyond the horizon.
It is important to note that Samuel F. B. Morse began work on the electric telegraph in 1832. He developed a practical system in 1844 and patented his technology (for Morse code) in 1849. Initially, Morse code only transmitted numbers. The transmission’s receiver would then have to use a dictionary to translate the numbers into words. But that proved to be tedious and cumbersome, so the code was expanded to include letters and even punctuation.
The original telegraph system had an apparatus on the receiving end that spat out a string of paper with indentations on it. Short indentations were called “dots” and the longer ones “dashes.” As telegraph users became more proficient with the code, they soon dispensed with the paper tape and deciphered code by ear.
As radio frequencies got longer and longer, international communication soon became possible and a need for an international standard code developed. In 1912, the International Morse code was adopted for all international communication.
