The 2013 NAWCC Ward Francillon Time Symposium and Special Exhibition of Spectacular Clocks, Watches, and Sundials
by The Pre-Eminent Master
Thomas Tompion (1639–1713)

California Institute of Technology Pasadena, California

7-9 November 2013

Linn Hobbs

John F. Elliott Professor of Materials Emeritus, Massachusetts Institute of Technology, Cambridge, Massachusetts

Time and Materials

  • Biography
  • Presentation
  • Further Information
  • Linn W. Hobbs was educated at Northwestern University (B.Sc. summa cum laude, 1966) and at Oxford University (D.Phil. 1972, as a Marshall Scholar), where he afterwards held an NSF Postdoctoral Fellowship and was subsequently elected a Research Fellow of Wolfson College, Oxford. He has held professorial appointments at MIT for the last 33 years and was the inaugural holder of the John F. Elliot Chair in Materials (1992-99) there. He has served as president of the Microscopy Society of America (1987), director of the Materials Research Society (1983-86), and director of the American Ceramic Society (2003-06). Among his many research and teaching interests are archaeological materials and the materials science of horological devices. He is a longtime restorer of antiquarian clocks, among them a Tompion & Bangor longcase clock for Wadham College, Oxford. He was made an Officer of the Order of the British Empire by Queen Elizabeth in 2001.
    The presentations following this introduction to the section of the Symposium entitled “Using Time” document the most recent technological innovations in timekeeping that parallel developments in the science of electricity and magnetism, solid-state physics, and atomic physics that began in the mid-19th century and have extended into this one. Electrical timekeeping, reviewed by James Nye in his lecture, began with London clockmaker Alexander Bain in 1840, but its realization would have to await the implementation, by Charles Wheatstone at King’s College London, of earlier experimental work of physicist Michael Faraday at the Royal institution, and the comprehensive understanding of electrical and magnetic phenomena provided by Cambridge physicist James Clerk Maxwell in 1861, which he mathematically codified into his eponymous classical equations that govern the induction and electromagnet technologies used in electric clocks and slave displays. The quartz oscillator timekeeper, whose evolution is described by David Rooney in his lecture on the quartz revolution, derives from the discovery in 1880, by the French physicist brothers Paul-Jacques and Pierre Curie, of the phenomenon of piezoelectricity, a property exhibited by certain non-centrosymmetric crystalline substances, like quartz, that deform elastically in response to an imposed electric field (electrostriction response) and conversely generate an electric potential when elastically stressed (piezoelectric response). The first quartz crystal oscillator was built by physicist Walter G. Cady in 1921; radio frequency control followed in the early 1920s, and the first quartz crystal clock in 1927, built by radio engineers Warren Marrison and Joseph W. Horton at Bell Laboratories. The atomic clock, the subject of Thomas O’Brian’s lecture, utilizes energy-quantized transitions between electron orbital energy levels within atoms themselves, which were understood only after the application of quantum wave mechanics—developed in 1926 by Austrian physicist and Nobelist Erwin Schrödinger—to the configuration of atomic electrons and their interaction with the nucleus. The principle of the clock’s operation draws on a magnetic-field resonance method, invented by physicist and Nobelist Isidor I. Rabi in the 1930s, for determining the transition energies between magnetic states of nuclei by simultaneous application of a magnetic field and radio-frequency electromagnetic modulation, a principle that is also applied in magnetic resonance imaging. The first atomic clock was constructed in 1949 at the National Bureau of Standards (now NIST), and the first 131Cs clock at the National Physical Laboratory in the UK in 1955.

    Most methods for measuring quantized time—the division of time into identical units repeating at a fixed frequency (rate)—are based on the principle of resonance, the ability of a device to both absorb energy at that fixed frequency and re-emit it at the same frequency with high efficiency. For a pendulum clock, the resonant frequency is determined by the simple-harmonic rotational motion of a constrained mass (the pendulum) subject to a uniaxial gravitational force, which absorbs a mechanical impulse at its resonant frequency and releases mechanical potential energy (through the train via the escapement) at the same frequency. In a quartz clock, the resonance is the natural mechanical vibration mode of a quartz crystal—which depends on its size and crystalline orientation. An alternating electrical voltage (kHz to MHz) induces the mechanical vibration via electrostriction, and the piezoelectric response of the deforming crystal in turn generates an alternating radio-frequency electric current of the same frequency that sustains the mechanical vibration and whose frequency can be read out. In an atomic clock, the resonance is the electron energy transition between two barely different electron orbital states of an unpaired atomic electron, split in energy by interaction with the magnetic field of the atom’s nucleus and identically established in a large number of identical atoms (e.g., specific hydrogen, cesium, or rubidium isotopes in the gaseous state). The state transitions are induced by microwave (GHz) electromagnetic radiation, and the associated microwave absorption provides an oscillator feedback loop that locks the (readable) microwave frequency onto the electron-state transition. These new paradigms for timekeeping have brought both increased precision and made accurate timekeeping more pervasive in our lives.

    The oblique reference in the title of this Introduction to “Time and Materials,” a customary basis for invoicing constructional projects, reminds us that the actual materials used for a given timekeeping methodology (or any technology)—their availability, processing, and ease of manipulation—have a great deal to do with paradigm shifts in these (or any) technological applications. Such materials considerations have always been important in the evolution of the mechanical clock, three examples being: the shift from iron to brass metallurgy in the late 16th century, which brought forth the domestic clock; the shift from machined cast brass to stamping of wheels and frames from rolled-brass sheet in the early 19th century (begun by Joseph Ives, John Birge, and Chauncey and Noble Jerome before 1837) that finally brought domestic clocks within the reach of most populations; and the late 20th-century introduction of microelectronic quartz oscillators to clocks and watches. The first American production of rolled-brass sheet, from cast ingots of brass formed by direct fusion of separately smelted copper and zinc metals, was begun by Abel and Levi Porter in Waterbury, CT, in Connecticut’s central Naugatuck Valley, starting in 1802, but few in the horological community may realize that the motivation was the demand for gilt buttons on military uniforms, a demand exacerbated by the subsequent War of 1812 between Britain and the U.S. Fewer still appreciate the prophetic irony that the principals who began the Connecticut rolled-brass revolution included the same Porter family, who contracted with Eli Terry in 1807 for mass-production of 4,000 wooden-movement clocks in three years, and a brother of Waterbury’s most celebrated traditional clockmaker, Mark Leavenworth. A second button-making firm began rolling brass in 1824 and spun off the Waterbury Clock Company in 1849 (later to become Timex).

    This 19th-century materials processing innovation enabled a pervasive change in clock design and affordability that persisted well into the mid-20th century, until another materials-driven revolution—the quartz clock and watch revolution—popularized a new timekeeping paradigm with the advent of materials processing for microelectronics in the 1970s, and the global positioning system—with its necessary reliance on the precision afforded by atomic clocks—made pervasive yet another paradigm shift at the turn of the 21st century, now available to every smartphone user.

    Further information coming soon