At first glance, the Stroh horn violin most closely resembles a brass instrument, rather than the family of violins from which it is descended. In fact, the Strohviol was considered to be cutting-edge science of the late Victorian era, and is the predecessor of the modern-day electric violin. With the use of a metal horn and resonating chamber, Stroh violins could better direct their sound into a recording horn, back in the days of wax cylinders and basic shellac recording technology. Chances are, if you listen to a violin recording dating before 1925, you're hearing a Stroh violin.
The Stroh horn violin is named for German inventor Johann Matthias August Stroh (1828 - 1914), who made his living in London as a telegraph and mechanical engineer, having gotten his start as a watch and clock maker (read more here). Stroh made a huge impact on the musical world when he invented the Strohviol in 1899 (Patent GB9418, titled "Improvements in Violins and other Stringed Instruments"). The Stroh violin is constructed of wood and aluminum, with two horns – a large one to project towards the audience or recording device, and a smaller one to allow the musician playing to monitor their own sound. This phonographic violin inspired other mechanically amplified stringed instruments in the early 1900s (example here).
So you might be wondering how a Stroh violin can even sound like a violin. All that seems to remain of the classic violin shape is the neck of the instrument, the strings that run from scroll to chin rest, and the bridge. Apart from that, it looks like a large metal horn. The key to the resonance is in how the bridge transmits the vibration of the strings into the diaphragm and then through the horn. This works in much the same way that a gramophone record needle amplifies vibrations through a horn (see video explanation here). This technological innovation in the design of the violin allowed for much louder mid-range and directionality in playing, which was a wonderful solution for the recording limitations of the day.
The horn design not only allowed for easier recording of the violin, but also helped the Strohviol to compete in volume with jazz band instruments of that time, making it a popular instrument among performing and busking musicians. The desirability of this advantage was reflected in its price tag. In 1911, a Stroh violin could sell for somewhere between 9 and 12 guineas (compared to 2 guineas for a standard violin), making it quite an expensive instrument for the time (see more). Some would describe the sound as more reedy compared to that of a standard violin, rather than tinny as might be expected from an instrument constructed mostly of metal.
Although the Strohviol began to lose popularity in the late 1920s with the advent of microphones and electric violins, it made a comeback in the Bihor region of Romania by Roma fiddlers as a vioară cu goarnă – "violin with horn" – where it gained new life as a Balkan folk instrument, and is still in use today (read more on this here). While Stroh violins fell out of fashion in much of Europe and the United States, this invention influenced the development of many other instruments still popular today, including the electric violin (for example, the Beauchamp Electro Violin of the 1930s) as well as the resonator guitar (with John Dopyera's 1928 patent referring directly to Stroh's 1899 patent).
While it's no longer quite as popular as in the early 1900s, the Stroh violin is still featured in popular music when musicians want to achieve that older sound. Some examples of artists who have featured a horn violin include Tom Waits, Bat For Lashes, and They Might Be Giants (with "I Can Hear You" and a few other songs on a wax cylinder). The Strohviol has also lately been adopted as a Steampunk instrument (a fashion that celebrates Victorian-era industrialism).
The Stroh horn violin excelled during a liminal time, just after the debut of recording technology and before the widespread use of electricity. This precursor of the modern electric violin has managed to make waves in the musical world, while surviving to the present day. If you listen carefully, you just might hear a Strohviol as you round a street corner. Maybe you'll be the one playing it!