We had tried several springs from hardware stores but they were all too stiff. This one works well for physics experiments
Good spring in the 200 - 500 grams range.
Very good spring, with an elastic constant of about 19 N/m. The spring has some residual force, about 100 grams are needed to start the stretching. In the 200 grams - 500 grams range this spring produces a perfect linear graph of force versus elongation.