General Description.—The rubber pendulum is one of two devices (Wiegand, Trans. Inst. Rubber Ind., 1, 141 (1925)) which, by employing the Joule effect, constitute rubber heat engines in that they continuously convert heat into mechanical work. In Fig. 1 is shown the original pendulum. It consists of an ordinary pendulum of slow period fitted with a rubber band, one end of which is attached to the bob; the other to the upright support. This rubber band is stretched to four or five times its original length. Behind the upright is a metal shield so arranged that when the bob has reached the extremity of its swing the rubber band is clear of the shield, during the rest of the oscillation being in its shadow. Behind the pendulum and shield is an electric heating element with a copper reflector. The pendulum is started by displacement from the center towards one or other extremity. As this is done the rubber band is increased in length. At the extremity of the oscillation the stretch band is exposed to the radiant heat from the element, the Joule effect is brought into play and the band tends to shrink, thus pulling back the bob. Directly the band moves back within the shadow of the shield it cools, relaxes, and so allows the bob to swing out to the other side. Thereupon the band is once more heated up, contracts and so repeats the oscillation, which continues as long as the heat energy is supplied. When the electric current is turned off the pendulum dies down.