The wire can contract up to 8%-10% of its length. For longer lifetime (greater than 1,000,000 cycles), you should restrict the contraction to only 6% of its length.

Contraction and relaxation depend solely on the temperature of the nitinol alloy wire. Any method of heating and cooling may be used. An easy way to heat the wire and one that is commonly employed is passing an electric current through it. Nitinol wire has a high resistance, approximately 1.25 ohms per inch for the 6-mil wire. The resistance of the wire to the electric current generates sufficient heat (ohmic heating) to bring the wire through its transition temperature.

Nitinol wire usually has a counter-force applied to it in the opposite direction of its contraction. The counter force resets, or stretches the wire back to its original length when in the low temperature phase. This is called the bias force.

If the nitinol wire is brought to its transition temperature without a bias force it will contract, however, when it cools it will not return to its original length. Consequently, without a bias force, when the wire is reheated no further contraction will take place. In most applications a bias force is applied to the wire constantly. Figure 2 illustrates two methods of applying a bias force, a spring and a static weight.

The speed and strength of the wire contraction depend upon how fast and how high the temperature of the wire is increased. For example, 400 mA of electrical current through the 6-mil nitinol wire will produce a maximum pull of 11 ounces and full contraction in one second.

Reaction time can be faster, in the millisecond range. To achieve this high current short duration pulses are used. When doing this consideration must be given to the mass and speed of the material to move. The faster you move a given mass the greater the inertia that must be overcome. If the inertia becomes greater than 6 lbs. for the 6-mil wire, it will snap.

Full contractive force is produced at the beginning of a cycle. In contrast to standard electrical solenoids that develop full strength near the end of their cycle.