for the Royal Society. In 1721, 's Gravesande became involved in a public controversy over whether the German inventor Johann Bessler, known as Councillor Orffyreus, had created a genuine perpetual motion machine. 's Gravesande at first argued for the feasibility of perpetual motion based on the conservation of the scalar quantity mv (mass multiplied by speed), which he erroneously believed was implied by Newtonian mechanics. However, in 1722 he published the results of a series of experiments in which brass balls were dropped from varying heights onto a soft clay surface. He found that a ball with twice the speed of another would leave an indentation four times as deep, from which he concluded that the correct expression for the "live force" of a body in motion (currently called "kinetic energy") is proportional to mv2. Even though those results invalidated his original argument for the feasibility of perpetual motion, 's Gravesande continued to defend Bessler's work, claiming that Bessler might have discovered some new "active principle" of nature that allowed his wheels to keep turning. Similar views were defended at the time by Gottfried Wilhelm Leibniz, Johann Bernoulli, and others, but the modern consensus is that Bessler was perpetrating a deliberate hoax. Russian Tsar Peter the Great was interested in Bessler's wheel and sought 's Gravesande's advice on the subject. 's Gravesande communicated his results on the impact of falling weights to Émilie du Châtelet. Similar observations were published independently by Giovanni Poleni. The interpretation of 's Gravesande's and Poleni's results led to a controversy with Samuel Clarke and other Newtonians that became a part of the so-called "vis viva dispute" in the history of classical mechanics. 's Gravesande's ring 's Gravesande is also remembered for his invention of a simple experiment demonstrating thermal expansion, which has been used in physics education since. This is known today as "'s Gravesande's experiment" or "'s Gravesande's ring". The apparatus consists of a small metal ball on a chain or handle, and a metal ring on a stand. The ring is just big enough so that when the ring and ball are at the same temperature, the ball fits through the ring. However, if the ball is heated by dipping it into boiling water or playing the flame of a spirit lamp over it, the metal will expand, and the ball will no longer fit through the ring. When the ball has cooled