Isaac Newton (1642–1727), considering collisions, first described momentum as the product of mass and velocity, but he called it “the quantity of motion.” Energy took 150 years from the first statement of principles until the terminology was worked out. Collisions also inspired the Dutch physicist Christiaan Huygens (1629–1695), who wrote that in the collision of two perfectly elastic spheres the sum of what we today call kinetic energy would not be changed by the collision. The German scientist Gottfried Wilhelm Leibniz (1646–1716) gave the name vis viva in 1695 to kinetic energy. But how could the conservation of vis viva be extended beyond elastic collisions? Finding the answer to this question took over 150 years!

An important contribution was made by Benjamin Thompson (1753–1814). Thompson was born in Massachusetts, but because he opposed the American Revolution he left for England and was knighted by King George III and given the title Count Rumford. While in America he spied for the British. While in England he spied for the French and was a counter spy for the British. He moved to Bavaria, now part of Germany, and became Minister of War, among other duties. Because he ran an orphanage and wanted to save money he studied heat and invented many items, like an efficient stove and a coffee percolator. A long series of experiments led him to conclude in 1798 that thermal energy was nothing more than the vibratory motion of what we know today as the atoms that make up the material.

About twenty years before Rumford’s work the French scientists Antoine-Laurent de Lavoisier (1743–1794) and Pierre-Simon Laplace (1749–1827) showed that heat produced by a guinea pig after eating was very close to the heat produced when the food was burned. The development of steam engines by James Watt and others stimulated studies of the relationship between work done and heat produced and how to make engines more efficient. Around 1807 the word “energy” was used with its modern meaning.

In 1842 a German physician, Julius Robert von Mayer (1814–1878), proposed that all forms of energy are equivalent and that the sum of all forms is conserved. He wrote in general, qualitative terms, although in later essays he included quantitative evidence based on the work done when a gas was heated. But his work resulted in little recognition until the end of his life.

About the same time, a British amateur of science, James Prescott Joule (1818–1889), began a series of experiments designed to determine the relationship between work done and thermal energy increase that resulted in heat transmitted to the outside. He explored electric generators, the compression of gasses, and stirring water. His experiments lasted eighteen years. As he continued to publish his results they were taken more and more seriously.

The German physicist and physiologist Hermann von Helmholtz (1821–1894) developed a mathematical description published in 1847 that showed precisely how energy was conserved in many fields including mechanics, thermal energy and heat, electricity and magnetism, chemistry, and astronomy. With his results the scientific community recognized the great achievement of Rumford, Mayer, Joule, and others and fully accepted energy conservation.