a chamber 10 has a passage 12 through which a piston or extensible member 14 is slidably received . an expandable medium 16 , such as a wax , fills the chamber . the wax expands significantly as it changes phase from solid to liquid . wax , for example , commonly increases 12 - 15 % as it changes phase isothermally from its solid to liquid state . a temperature changing means 20 , preferably a peltier effect thermoelectric heating / cooling chip , selectively adds and removes heat from the expandable medium in the chamber . when connected with a source of electricity of one polarity , the peltier effect chip heats its surface 22 closest to the chamber to transmit heat energy into the wax . when connected with the opposite polarity , the peltier chip draws heat from its face against the chamber and discharges the heat through cooling fins 24 on an opposite face . a temperature control means 26 controls the peltier chip to hold the expandable medium substantially at its melting temperature . when thermal energy is applied to room temperature wax , the wax retains its solid form but increases in temperature until it reaches its melting point , e . g . 125 ° f . the additional energy necessary to change from the solid to liquid phase is supplied by the application of additional thermal energy . however , the absorbed thermal energy causes an isothermal phase change rather than increasing the temperature of the wax until the phase change is completed . if additional thermal energy is applied after the phase change , the liquid wax would increase in temperature . when thermal energy is removed , the liquid wax isothermally solidifies . in this manner , the wax expands and contracts about 12 - 15 % as heat is added to or removed from the wax which is held at its melting point temperature . the temperature control means 26 includes no direct thermal element in the preferred embodiment . rather , it controls the temperature of the expandable medium by monitoring the volume . the expandable medium is heated to and held at the melting point temperature . a small additional amount of heat is added to a small fraction of the expansion , e . g . to change a small percentage of the expandable medium to its liquid state . this marks the equilibrium retracted position . a first cam operated switch 30 that is received in a first recess 32 in the piston closes each time the piston starts to retract and opens whenever the piston starts to extend . when the switch closes , indicating that the small percentage of the liquid phase is starting to solidify , the first switch 30 closes conducting electrical current to the heating means to add additional heat energy into the expandable medium . when the retracted equilibrium position is again attained , the cam operated switch 30 opens and the application of heat is terminated . to expand or retract the piston , voltage of an appropriate polarity is applied to a control lead 34 . to expand the piston , a first polarity voltage is applied to the lead and conveyed through a second cam operated switch 34 - 36 to the peltier chip . voltage of the first polarity causes the peltier chip to pump heat into the expandable medium effecting the phase change , expansion of the medium , and extension of the piston 14 . when the piston has been extended its full range , the second cam operated switch engages the first recess 32 and opens to terminate the supply of power to the peltier chip . in the extended position , the second cam switch 36 engages the first recess 32 and opens . the second switch 36 closes each time the piston starts to retract , supplying more heating polarity voltage and opens when it returns to the full extended position . to retract the piston , the opposite polarity potential is applied to the control lead 34 . the opposite polarity control voltage is conveyed through the second switch 36 to the peltier chip 20 to operate it in a heat removal mode . the heat is removed until the cam of the second switch 36 falls into a second recess 32 in the piston , opening the second switch and stopping the application of second polarity potential . the first switch 30 interacts with the first recess 32 in the piston to maintain the piston in the retracted position . the speed with which the actuator extends and retracts is determined in substantial part by the rate at which heat is introduced into and removed from the heat expandable medium . the efficiency of the heat transfer is increased by increasing the surface area to volume ratio of the expandable medium , i . e . the chamber interior . in the embodiment of fig2 the chamber includes a plurality of thin channels that are cut or etched into a highly heat conductive material , e . g . copper or aluminum , which defines the chamber . preferably , each path is relatively thin in the direction of heat transfer . when the wax or other expandable material is relatively thin in the direction of heat transfer , the distance that the heat propagates through the wax is minimized . it might be noted that the heat conductivity of copper is about 280 times the conductivity of wax . the slow transfer of heat through the wax is a limiting factor on the rate at which the wax can change phase . flat , sandwich designs , as illustrated in fig2 and 3 , are particularly effective in this regard . the chamber 10 is also constructed with sufficient strength to accommodate the pressures developed in the actuator . pressures on the order of 30 , 000 psi can be attained . accordingly , the chamber walls must have sufficient strength to withstand such pressures . relatively thin slabs of copper , e . g . 0 . 020 inches , with relatively small internal chambers , e . g . 0 . 020 inches , cross tied for strength have been found to be effective . a flat exterior surface maximizes the contact area between the heat conductive chamber and a flat peltier chip to improve the efficiency with which heat is moved . other surface and peltier chip shapes may be utilized as are appropriate . in the embodiments of fig2 and 3 , a central piston reservoir 40 is fed by a tree or river - like pattern of contributory paths 42 . that is , relatively small paths merge into larger contributory paths which merge again into still larger paths , etc . alternately , linear tapered paths may extend in parallel from the piston reservoir . this gradual expansion of paths flowing into the central piston reservoir is of particular importance during piston retraction . the thinner paths tend to cool first such that the wax cools or retracts from the inner most end first . if the wax were to start contracting at the center , it could conceivably form a plug such that the contraction of wax upstream only draws a vacuum rather than assisting in retracting the piston . optionally , a return spring 44 may be provided for assuring that the piston retracts into the cylinder as the wax contracts rather than allowing a vacuum to be drawn . as another option , thermal insulation 46 may cover exposed surfaces of the chamber to minimize incidental heat transfer . limiting heat transfer to the heating and cooling means 20 improves controllability . in the embodiment of fig4 the heating / cooling means includes a tube 50 passing through the expandable medium 16 in the chamber 10 . the chamber connects with the end of the piston . a thermal liquid is pumped through the tubing to heat the wax to effect a solid - to - liquid phase change or to cool the wax to effect a liquid - to - solid phase change . in the embodiment of fig5 the heating / cooling means 20 includes a resistive heating element 52 passing through the chamber to heat the wax . a cooling coil 54 selectively pumps a cooling fluid through the wax to solidify it . in the embodiment of fig6 the heating and cooling means 20 is a freon heat pump . a compressor 60 compresses freon from its gas phase to its liquid phase , which compression adds heat to the liquid freon . a heat / cool control valve 62 determines whether the heat pump heats or cools the wax . in the heating mode , the hot freon passes through a heat exchanger coil 64 in the wax in the reservoir . the liquid freon is converted into a gaseous freon state at a restrictor 66 and discharged into a heat exchanger 62 . the gaseous freon is returned to the compressor and the cycle repeated . in the cooling mode , the compressor pumps the hot liquid freon to the heat exchanger 62 where it is cooled towards room temperature . the phase change at the restrictor 66 drops the temperature of the freon substantially as it passes into its gaseous phase . the heat exchanger coil 64 is cooled by the phase change withdrawing heat from the wax . one or more of the piston and cylinder combinations may be connected with a means 70 for converting the reciprocating movement into rotary motion . in the illustrated embodiment , the reciprocating to rotating conversion means includes a crankshaft 72 that is driven by two identical piston / cylinder combinations . each piston cylinder combination is rotatably connected by a bearing 74 with the crankshaft and pivotally connected with ground at the other end by a bearing 76 . a tachometer 72 monitors rotation of the crankshaft to indicate the beginning of the piston extension and retraction cycle portions . the temperature control means 26 alternately supplies voltage of the first and second polarity to the peltier chips 20 , out of phase . of course , the crankshaft may be driven by a single actuator or a large plurality of actuators . the invention has been described with reference to the preferred embodiment . obviously , modifications and alterations will occur to others upon reading and understanding the preceding specification . it is intended that the invention be construed as including all such alterations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof .