This spring is made from shape memory alloy (SMA).  SMA is a smart material which, as its name suggests, has a memory. The most common SMA is an alloy (mixture of metals). By means of special heat treatment, a piece of SMA can be made to 'remember' a shape. For example, a length of wire can be made to remember that it should be straight at temperatures above 70°C. If you bend this wire at normal room temperature into the shape of a paper clip, it stays bent and will continue acting as a paper clip. However, if you place it in a glass of water whose temperature is above 70°C, it immediately straightens out! When cool, it remains straight until it is bent again.

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.