Ten Physicists in Order of Birth Year, From Galileo's Thermoscope to Lawrence's Cyclotron

Around 1592 Galileo built an open thermoscope, a sealed glass bulb and tube whose liquid level rose and fell with temperature. It was the earliest known device to register temperature change, yet it had no fixed scale and responded to air pressure as well as heat, which is exactly the problem the next stop solves.

In 1714 Fahrenheit produced the first thermometer to use mercury, sealed in a glass tube with a reproducible fixed-point scale. Because the scale could be reproduced, readings made in different places could be compared, and the temperature scale he devised still carries his name.

Joule used a falling weight to drive a paddle wheel through insulated water and measured the heat produced. The fixed proportion between work and heat, the mechanical equivalent of heat, gave the first law of thermodynamics its experimental foundation and helped overturn the caloric theory.

Working with the chemist Robert Bunsen, Kirchhoff showed in 1859 that each element, when heated, produces its own distinctive pattern of spectral lines. The pattern let them identify the new elements caesium and rubidium, and it later gave astronomers a way to determine what distant stars are made of.

Around 1866 Maxwell formulated, independently of Ludwig Boltzmann, a kinetic theory of gases in which molecules at a given temperature move at a range of speeds distributed according to a definite law. Heat and temperature became statistical properties of molecular motion, which gave the thermodynamics of the earlier stops a mechanical foundation.

Morley was a chemist known for exacting laboratory measurements, including a precise determination of the atomic weight of oxygen. In 1887 he brought that measurement discipline to the Michelson-Morley experiment, which was designed to detect motion through the hypothesized luminiferous aether.

Michelson built the interferometer, which splits and recombines a beam of light to detect tiny differences in the distance it travels along two paths. The 1887 experiment with Morley found no motion of the Earth through the aether, a null result that helped clear the way for special relativity and earned him the 1907 Nobel Prize in Physics.

Thomson discovered the electron in 1897. Around 1912 to 1913 he built an early positive ray apparatus that passed charged particles through electric and magnetic fields to sort them by mass, producing the first mass spectra and the first evidence that some elements exist as a mixture of isotopes.

Aston, Thomson's assistant, refined the positive ray apparatus into the mass spectrograph reported in 1919, which focused ions of the same mass regardless of their velocity. With it he identified 212 naturally occurring isotopes and formulated the whole number rule, winning the 1922 Nobel Prize in Chemistry.

Beginning in 1930 at the University of California, Berkeley, Lawrence developed the cyclotron, the first circular particle accelerator, which sends charged particles along an outward spiral using a fixed magnetic field and an alternating electric field. His design became the model for later accelerators, and he received the 1939 Nobel Prize in Physics for it.