The Stroh violin or Stroviol is a type of stringed musical instrument that is mechanically amplified by a metal resonator and horn attached to its body. The name Stroviol refers to a violin, but other instruments have been modified with the amplification device, including the viola, cello, double bass, ukulele, mandolin, and guitar. John Matthias Augustus Stroh, an electrical engineer in London, invented the instrument in 1899. Description The Stroh violin has a horn at the end of the fingerboard to project the sound to an audience or recording horn, and often a smaller monitoring horn that the performer placed at their ear to hear what was being played more distinctly. The Stroh violin is much louder than a standard wooden violin, and its directional projection of sound made it particularly useful in the early days of phonographic recording. Wooden violins recorded poorly with the early acoustic-mechanical recording method, and the Stroh violin improved this by producing a fuller, louder sound. Stroh violins were common in recording studios, but became rarer after record companies switched to the new electric microphone recording technology in the second half of the 1920s. Invention On 4 May 1899, Stroh applied for a UK patent, GB9418 titled Improvements in Violins and other Stringed Instruments which was accepted on 24 March 1900. This described the use of a flat metal (other materials are also mentioned) diaphragm in the voice-box (reproducer) of a violin to mechanically amplify the sound. On 16 February 1901 he applied for a second UK patent, GB3393 titled Improvements in the Diaphragms of Phonographs, Musical Instruments, and analogous Sound-producing, Recording and Transmitting Contrivances which was accepted on 14 December 1901. This effectively extended the first concept to now use a conical resonator with corrugations at its edge, allowing a more 'rigid' diaphragm. His failure to register his inventions in the USA allowed John Dopyera and Geo Beauchamp to subsequently obtain US patents for the tricone and single cone designs used in National brand instruments. Usage The Stroh violin was an expensive instrument: in 1911 it was offered by the London dealers Barnes & Mullins for nine guineas (£9.45, then equal to $37.80) or twelve guineas (£12.60/$50.40) at a time when a reasonable factory violin could be had for two guineas. It was listed as being especially suitable for use in small theaters and music-halls. In 1920s Buenos Aires, Julio de Caro, a renowned