Abstract: This invention relates to apparatus for the conversion of thermal energy into mechanical motion. Transducers, such as nickel-titanium alloys, are employed which change their configuration and pliancy upon the application of stimulus such as heat. In the absence of a suitable stimulus, they are pliant and easily flexed. However, when stimulated they tend to assume a predeterminable configuration and then become rigid. Apparatus utilizing these phenomena convert the energy of physical transformation into useful mechanical work.Several embodiments are described. In one, the transducer is belt shaped and drivingly connected to several pulleys. Heat stimulus is applied to the belt locally adjacent at least one of the pulleys. The resultant straightening forces, inherent in an alloy such as nickel-titanium, cause rotation of the belt and hence the pulleys. In another embodiment, the transducers are opposingly connected to a swinging member so as to produce reciprocating motion when selectively stimulated.