These remarkable springs, based on an alloy of nickel and titanium (Nitinol or NiTi), contract when heated.
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).
10mm x 5.5mm with a 4mm internal diameter.
Video below shows both PAC-SW3 and PAC-SW7.