The nail violin is a musical instrument was invented by German violinist Johann Wilde in 1740. It consists of a semicircular wooden soundboard with iron or brass nails of different lengths arranged to produce a chromatic scale when bowed with a violin bow.

Nail Violin

Materials
• Piece of 1/2" plywood, 12" x 6" or larger
• 5 nails: Three 12d, One 10d, One 8d
• Violin or cello bow, or general instrument bow

Step 1: On the plywood, mark out an arc by using a compass set to 5" (or do it freehand).
Step 2: Starting 1" from the edge of the wood, pound in nails every 1-1/2" along the arc–putting the largest nails at the top and the smallest at the bottom. Don’t hammer them too deep. Tune the instrument: raise the pitch of any nail by hammering it in; lower the pitch by carefully pulling out a little with the hammer's claws.  If the nail will not pull out, use a metal file to gently file away a small notch near the base of the nail.
Step 3: Tune to: D, E, G, A, B

Variations:  Of course, you can add many more nails to the instrument and increase the number of tones.  For additional resonance, glue the plywood base to a wood cigar box. 

Plugging in:  A piezo pickup placed at the center of the plywood will do well for amplifying the nail violin.

Playing the Nail Violin
Bowing the nails nearest the nail heads gives you the best sound.

Here's some other nail violin examples I've found: