0
A HEAT ENGINE RUN BY RUBBER E. G. COX The University, Leeds, England
FORa fixed extension not exceeding about 350 per
to cool. By suitable adjustment of the heated area cent the tension in a strip of rubber is approximately continuous rotation in one direction can be maintained. proportional to the absolute temperature over the About 60 ordinary elastic bands a t 300-400 per cent range 200'-400°A. This result, with its obvious extension are used as spokes; a rather large extension is analogy to the simple gas law, provides direct experi- necessary to prevent sagging of the rim. I n order t o mental support for the kinetic statistical theory of equalize tensions in opposite spokes each band was rubber elasticity, for it shows' that, appr~ximat~ely,stretched across a diameter and the hub was clamped over the range stated, the internal energy of rubber a t to the spokes a t the center of the wheel by a washer and a constant temperature is independent of its extension nut. Final balancing of the wheel was achieved by and that the increase in tension with stretching is due adjusting small pieces of plasticine on the rim. almost entirely to decrease of entropy. The "perfect Heat is applied by placing a 60-watt lamp bulb close rubber" is therefore an interesting alternative to the to the lower spokes and somewhat to one side of the perfect gas as a subject for exercises in elementary vertical through the axle; a reflector is placed opposite thermodynamics; as for a gas, a simple kinetic explana- the lamp behind the wheel so as to increase the heating tion of the facts can be given, but rubber has the advan- effect by throwing back some of the radiation which tage of being more easily used as the working substance has passed through the spokes. The wheel then turns in a simple heat engine suitable for lecture demonstra- at a rate whirh can be regulated to some extent by "adtions. Such an engine was apparently first described vancing" or "retarding" the heater but which is of the by Wiegand, who constructed a pendulum and a order of magnitude of one revolution per second; we rotary "rubber m ~ t o r , "in~ both of which continuous have not had occasion to run it continuously for more motion was achieved by the alternate heating and than two hours but there seems no reason why it should cooling of stretched rubber. It is, however, not an not run for much longer periods. It ran successfully easy matter to obtain consistently good results with as first made with materials which happened to be these devices, and I have found that.the considerably available so that there is apparently nothing very simpler arrangement described below, which is similar critical about the details of design or construction. to the muscle motor of P r i ~ r is , ~ much more satis- An improvement suggested to me by Mr. D. W. Saunfactory. ders is to coat the surfaces of the rubber spokes with The motor consists of a wheel with a ball-bearing carbon black to increase the absorption and radiation hub, rubber spokes, and a rigid light metalrim 20 cm. in of heat. diameter, arranged to rotate about a fixed horizontal We have also been able to produce rotation by the axle. If radiant heat is applied to the lower spokes of application of cold to the upper spokes instead of heat the st,ationary wheel they contract slightly, thereby to the lower ones; with the arrangement used (a stream raising the center of gravity of the rim above the fixed of air of a few square centimeters cross-section, cooled axle, so that the wheel turns, bringing fresh spokes by passing through carbon dioxide snow) the motion into the radiation and allowing those previously heated was slow and irregular, but with attention to details of the cooling arrangements better results could no I See, e . y., TRELOAR, I. R. G., "The Physics of Rubber Elas- doubt be achieved. ticity," Oxford University Preaa, London, 1949, Chapter 11. I am indebted to Mr. V. Balashov for assistance in WIEGAND. W. B.. T7an.v. Zmt. RubberInd.. 1. 16G5 (1925). . . the construction of this apparatus * Pmon, M. G. M:, Natzwe, 171, 213 (1953j. '
