Free The Big Questions: Physics by Michael Brooks Page A

for the uppermost rod to flip over as it reaches the top of the wheel, maintaining the imbalance. Unfortunately, this doesn’t happen: the weight distribution is such that it doesn’t quite flip. After one revolution, the weights return to their initial position, and everything is back exactly where it started – including the stationary wheel.
     

     
    To be fair to de Honnecourt, the reason for this was not clear until well after his time. The problem is that energy is transformed between two different forms. Because the rods have the potential to fall under the influence of gravity, they are said to have ‘potential energy’. If the wheel turns, some of this converts to the ‘kinetic energy’ of movement. However, after one cycle, the rods return to their initial position, and therefore must have exactly the same potential energy (which is due to their position) as before. Since there is no external source of energy, and the rods have the same potential energy at every turn, there is nothing to put energy into turning the wheel.
     
Energy is conserved
     
    By 1775, the Royal Academy of Sciences in Paris had had enough of perpetual motion. It issued a statement declaring that the Academy ‘will no longer accept or deal with proposals concerning perpetual motion’. And in 1841, scientists finally found a scientific principle to throw at perpetual motion seekers: the first law of thermodynamics.
     
    It was the first explicit statement of the conservation of energy. Leonardo da Vinci had suggested that, ‘Falling water lifts the same amount of water, if we take the force of the impact into account,’ but it took the German physicist Julius Robert Von Mayerto explore the matter properly and issue an edict. Energy, he said, cannot be created or destroyed.
     
    Not that he was taken seriously straight away: Von Mayer was told, for instance, to find some experimental evidence to back up this strange idea. This he did, by showing that the kinetic energy of vibration could be transferred to water molecules, manifesting as an increase in temperature. Once the point was proven, the principle was quickly accepted by physicists, and used to keep perpetual motion at bay. Motion takes energy, and the conservation of energy principle tells us that you can’t get more energy out of a closed system than is there in the first place. Since friction affects any and every mechanism, dissipating some of that energy as heat and sound, inventing perpetual motion machines of the first kind became a fool’s errand. Not that this put the perpetual motion seekers off. Around this time, the science of thermodynamics was giving them a whole new lease of life. Their goal? Perpetual motion machines of the second kind.
     
Miracle machines
     
    The second kind of perpetual motion machine is something that extracts heat energy from a reservoir, such as the air or the ocean, and converts it into mechanical energy. It certainly seems like a good idea. The oceans are so vast a resource that, if we could extract heat that would cause a one degree drop in ocean temperatures, it would supply something like the energy needs of the United States for half a century.
     
    The plausibility of this kind of machine is enticing. Indeed, creating an efficient steam-powered engine has been a human obsession since Hero of Alexandria created the ‘aeolipile’ in AD 1. This ball, that was set rotating by jets of steam, had no particular uses. However, subsequent inventions used steam turbines to turn spits, pump water from mines and power grinding pestles. None of them got anywhere near a truly useful efficiency, however. That efficiency came with James Watt’s steam engine, first demonstrated in 1765. It was a development of the engine invented by Thomas Newcomen, and raised the efficiencyenough to kick off the Industrial Revolution. The theory behind such engines, though, was still very much in development. The builders of steam engines were working on