In the early 1900s the navies of the world were quickly expanding their submarine fleets and with them the technology that they used to fight.
One problem submarines faced as they matured was how to communicate underwater. Radio communication was only a couple decades old and radio waves barely transmit underwater. To allow submarines to communicate with naval forts, each other, and surface vessels, the U.S. Navy tested an improvised “violin” for submarines that allowed communication at ranges of up to five miles.
The violins had a single string stretched between two large, metal rods connected to the hull of the submarine. A wheel with a rough edges spun in close proximity to the string. A Morse-code operator could tap a button that pressed the spinning wheel to the string, sending a vibration through the string, the rods, and then the submarine hull itself. This vibration would continue through the water to underwater microphones on ships and coastal installations. The tests began in 1913 with three violins being installed on three ships. There’s no sign that these violins were ever used in combat.
Today, submarines and sub killers hunt using sonar, so sound-emitting devices like the submarine violin would be too dangerous to use. Instead, modern subs use specialized antennas and very low frequency radio waves to communicate.