© Science Museum/Science and Society Picture Library
James Watt trained as a maker of mathematical instruments, but went on to devise a fundamental improvement to steam engine design in the 1760s with his separate condenser. Early Newcomen engines used steam at quite low pressure and condensed it in the cylinder to create a vacuum. Atmospheric pressure above the piston then drove it down on the power stroke. This alternate heating and cooling of the cylinder wasted expensive fuel. Watt improved efficiency enormously by condensing the steam in a separate chamber alongside, allowing the main cylinder to be kept hot at all times.