Loops on both ends of the spring Length from Loop to Loop: 4.5 in Length of Spring (without loops): 4 in Outer Diameter: 0.2270 in Inner Diameter: 0.1635 in The sturdy, 4 inch extension spring provides the perfect tool for studying motion of mass on a spring, Hooke's Law, potential energy, etc. It provides a visually and kinesthetically effective method for studying the physics of springs.