The five color-coded equal-length springs in this set have different spring constants: 25 N/m, 30 N/m, 35 N/m, 40 N/m, 50 N/m (±5%) These springs appear to be the same except for their colors. But, when equal masses are hung on them, each stretches a different amount. These extension springs are made of steel and are closed, requiring a slight initial force to separate the coils. The unstretched length of each spring is 30 cm and the approximate diameter is 1.4 cm. These springs are supplied with a white storage box with cardboard separators to keep the springs from touching each other.