The nail violin is a musical instrument which was invented by German violinist Johann Wilde in 1740.^[1][2] The instrument consists of a semicircular wooden soundboard, approximately 1.5 feet (46 cm) by 1 foot (30 cm) in size, with iron or brass nails of different lengths arranged to produce a chromatic scale when bowed.^[3]

Wikipedia: https://en.wikipedia.org/wiki/Nail_violin