HayEastdor 20PCS Small Extension Spring 1/4 Inch x 1-1/2 Inch SP9600 Stainless Steel Mechanical Springs with Hook Ends HE027

I use these in costume design, for securing moving parts. They're great because they're a standardized size, and I can easily get lots of them, and they're a perfect strength for adding interactivity to hand-driven props.I measured the tension of first separation of one of these to be about 4.4 newtons (about 450 grams of mass) and you can stretch it to be less, all the way down to 0.They come in a nice little plastic organizer bin. The ends are also bent into a full loop, as opposed to a half loop, which makes them easier to get into things as there's a little more leeway to twist and wrangle the spring onto whatever catch holds it.

Good quality springs that are strong for their size. Although the average user like me may not have a ton of applications for these springs, at less than $10 for 20 of them, they're a great item to have on hand.

These springs are very small, yet surprisingly strong. Measuring only 1/4" in diameter and 1.5 inches longit is definitely possible to attach two or three of them together to make a longer spring, provided there is sufficient tension to hold them together (or you can bend the ends closed more tightly.