I employ the sound of springs a lot in my music, particularly for my soundtrack work. To that end I've collected field recordings of many different-sized springs found on industrial and farm equipment. A few years ago I salvaged two springs from the door of a broken dishwasher, mounted them on a board and named the resulting contraption the 'Spring Thing.'
In my travels around the internet I found details of an instrument with much longer springs, played with a violin bow. This inspired me to attempt something similar, although mine is a far simpler in design. For this project I bought a zinc 'storm door' spring, which is used to keep screen doors closed against the weather. It's a half-inch wide and about a foot long unextended. Armed with this, a length of 1x2 pine, and some hooks and hardware, I set about creating the Spring Thing Mk II.
The storm door spring has a smaller spring threaded inside it for adjusting tension. The first order of business was to remove this, and after much trial and error I cut it out (along with a half-inch or so of the larger spring) with a Dremel cutting blade. At one end of the board I mounted a 2" hook, and at the other a 1/2" eye-hook. One end of the spring was threaded over the hook, then the free end was threaded with an S-hook and stretched toward the eye-hook with pliers. The S-hook made attaching the free end a lot easier, as the coils are exceedingly tight and difficult to thread anything into, so doing so while it's under tension would be nigh impossible.
I was able to stretch the spring to roughly twice its original length: not as much as I'd hoped for, but decent considering the strength of the spring. The lower end contacts the board and imparts some vibration to the wood. Because the opposite end is raised, there's room to mount a contact mic beneath the spring. I had good results about three-quarters of the way along. I've also done some sessions with two condenser microphones, one placed at either end. The sound of this spring is much brighter than the Mk I.
The 'striker' seen in the pic is a chromed rod salvaged from a faucet set. Its intended use is to open and close a built-in basin stopper. It has a nice weight to it, with a hammer-like metal disk at one end and a smooth surface for scraping along the spring. It almost seems designed for the job!