amplifier output to saturate in one direction or the other the amplifier includes a negative feedback circuit 260 which provides dynamic compensation for the servo loop the time constant of this feedback circuit is effectively the same as the heating and cooling time constant of the nitinol wire 70 the dynamic system gain i e the response to sudden changes is relatively low but the static gain is very high so as to assure high positioning accuracy the slide wire 224 is supplied with a d c voltage 8 125 volts from an integrated circuit voltage regulator 262 of conventional design this supply voltage also energizes a voltage dividing circuit including resistors 264 266 1 47k 6 19k and resistor 210 resistor 210 provides a bias signal which causes the position of the slide wire to be a short distance e g 5 of travel from the end of its stroke when the command signal is zero this avoids any problems with non linearity which might arise near the end of the slide wire stroke the junction between resistors 264 266 is connected through an isolating diode 268 to the base of a transistor 270 if the voltage at the wiper of the slide wire 224 exceeds a predetermined value indicating that the output lever 76 has reached a pre set limit position for contraction of.