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Introduction to the constants for nonexperts The following material is intended to give nonexperts insight into the general subject of fundamental physical constants. It is adopted from an article written by Barry N. Taylor in about 1971 for the 15th edition of the Encyclopaedia Britannica © 1974 , and is reproduced with permission. Although a considerable portion of the article is now out-of-date, the introductory and historical portions given here remain generally valid. Note, however, that the terms accuracy and uncertainty are used in the text in a manner that is not always consistent with current practice. This also applies to other terms such as parts per million and its symbol ppm, and the now obsolete terms atomic weight and molecular weight. To add a more modern flavor to the material, a fundamental constant experiment of current interest is also described: the determination of the fine-structure constant by means of the quantum Hall effect and calculable cross capacitor. Introduction 1900 - 1920 1920 - 1940 1940 - 1960 Current (index) Index of experiments described Elementary charge Oil drop experiments Using the Avogadro constant and the Faraday constant Fine-structure constant Quantum Hall effect and calculable cross capacitor Fine structure of atomic hydrogen Changes in the atomic energy states induced by irradiation with microwaves Free electron g factor Change in spin direction of trapped electrons with prealigned spins Ground state hyperfine splitting in atomic hydrogen Hydrogen maser Gyromagnetic ratio of the proton Precession frequency of protons in a magnetic field Magnetic moment of the proton Acceleration of protons in a magnetic field using radio-frequency fields Newtonian gravitational constant Deflection of a plumb line from the vertical Measuring the period of a pendulum's swing Ratio of the electron charge to the electron mass Deflection of beams of free electrons by electric and magnetic fields Ratio of Planck's constant to the elementary charge The photoelectric effect Short wavelength limit of the continuous x-ray spectrum Velocity of light in vacuum 16th-18th centuries: Handheld lanterns, astronomical measurement 19th-20th centuries: Reflecting light between a rotating mirror and a fixed mirror

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