concomitant variation in the volume of the displacement cavity 204 the biasing spring 214 is situated within the cylindrical bore 212 of the rigid restraint 213 coaxial with the plunger 211 the relative positions and dimensions of the plunger 211 the rigid restraint 213 and the biasing spring 214 are such that at equilibrium the biasing spring 214 exerts a force on the plunger 211 along a line coincident with its axis such that the displacement cavity 204 is in a state of minimum volume fig2 a a straight length of shape memory alloy wire 215 is situated in a position coincident with the axis of the plunger 211 one end of the shape memory alloy wire 215 is fixed to the rigid restraint 203 and electrically connected by connection 216 to the programmable pulse generating circuit 217 and the electric power source 218 the other end of the shape memory alloy wire 215 along with an electrical connection 219 to that end is connected to the end of the plunger 211 the shape memory alloy wire 215 and the biasing spring 214 are both of sufficient dimension such that when the shape memory alloy wire 215 is heated so as to induce phase transition and associated dimensional change it will pull the plunger 211 against the force of the biasing spring 214 so as to create a state of maximum volume within the displacement cavity.