The Thermo-Elastic Properties of Muscle

The thermal effects of applying a stress to a body were first studied by Weber (1), who found that when an iron wire was stretched a thermal effect was produced, and that the thermal change was proportional to the stress. Lord Kelvin (2) deduced the general equations of thermo-elasticity from the laws of thermodynamics, and proved that with stresses of the most general type the thermal effect is proportional to the applied stress, provided the material remains perfectly elastic. In a body which expands on being warmed, the effect of an extension is a fall of temperature, while a body which shortens on being warmed will show a rise of temperature on extension. It will be remembered that Engelmann found that some substances ( e. g. , catgut) containing doubly refractive particles contract on heating; such bodies, therefore, would be expected to show a rise of temperature on being loaded, and the experiments given below prove that muscles and rubber belong to this class. The phenomena of thermo-elasticity have been employed to demonstrate the stresses set up in structures on loading them; all that is necessary is to place the “warm” junctions of a suitable thermopile against the part it is desired to investigate, and then, on loading the structure in any desired manner, the magnitude and sign of the stress in that part can be determined at once from the deflection of a galvanometer. This method, which is quite reasonably simple to employ, appears to deserve a wider application than it has received. It could be applied to a variety of substances under a variety of conditions, e. g. , to the materials employed in the construction of aircraft, ships, guns, etc., in which considerable stresses have to be borne by structures which have to be kept as light as possible. It might even be employed in physiology to determine the distribution of stresses in the skeleton, and it would certainly be of interest to make an extensive investigation of the thermo-elastic properties of elastic colloidal materials.