This nickel/titanium (NiTi) spring is made from shape memory alloy.
At room temperature this spring is soft enough to pull out to approximately 50mm by applying a small force.
When heated to 70°C by passing an electric current through it (or by other means such as hot water or a heat gun, etc), the spring contracts to its original length with a useful pulling force (equivalent to lifting a 0.5kg weight).
Spring weight: 1.1g
Dimensions:20mm (when closed), 5.5mm external diameter, 4mm internal diameter.