Robert Hooke - Artificial silk, cells, elasticity, astronomy, architecture, combustion, and more

Illustration of the Monument, London, a structure built by Sir Christopher Wren based on designs by Robert Hooke to commemorate the rebuilding of London following the Great Fire in 1666.  The monument was designed as both a landmark and an immense vertical telescope with a twenty foot deep underground laboratory.

Illustration of the Monument, London, a structure built by Sir Christopher Wren based on designs by Robert Hooke to commemorate the rebuilding of London following the Great Fire in 1666.  The monument was designed as both a landmark and an immense vertical telescope with a twenty foot deep underground laboratory.

The concept of a synthetic fiber was first suggested by none other than Robert Hooke, another Englishman of notoriety from the 17th century.  Hooke built some of the earliest telescopes to observe the planets, and was among the first to flip the telescopes around and use them as microscopes to study plant and animal cells.  In fact, he was the one that coined the term “cell” to describe the basic unit of life.  His contribution to the story of electrospinning was in a passing reflection that if “very quick ways of drawing [a synthetic fiber] out into small wires for use could be found,” then artificial silk could be produced.

Hooke had a number of other contributions worth mentioning in their broader significance to mechanics and science.  Through a series of careful experiments, Hooke discovered that the extension of a spring is proportional to the force applied.  In other words, doubling the load on a spring doubled the stretch (displacement).  This observation is the basis for what is now known as Hooke’s law, which is the fundamental equation for elastic bodies.  Put simply, essentially all materials that we regard as “solids” follow this elastic behavior for small displacements.  The term “elastic” means that the original shape is recovered after deformation.  In contrast, a “plastic” object remains deformed.  Engineers vastly prefer elastic ones because the equations are simpler (linear!), and if something deforms plastically it is not long for this world.

Frontispiece of Robert Hooke’s De potential restitutiva, showing set-up of spring experiments that revealed first insights into elasticity of materials.

Frontispiece of Robert Hooke’s De potential restitutiva, showing set-up of spring experiments that revealed first insights into elasticity of materials.

It may be somewhat revealing of Hooke’s personality that he described discovery of the law of elasticity in 1660 using the anagram, “Ceiiinosssttuv.”  He didn’t publish the solution, “Ut tensio, sic vis,” meaning “As the extension, so the force” until 1678, due to the commercial significance of the finding for the watch industry.  During the 18 years between his cryptic pronouncement and later revelation, he developed the balance spring, or hairspring, which enabled the accurate timekeeping of a small timepiece (a watch) for the first time.  Bitter disputes about priority of the invention with Christiaan Huygens later followed, but were later resolved favoring Hooke due to a note related to a 1670 demonstration of the balance spring to the Royal Society.  It is interesting to note that this strategy of publishing a cryptic anagram to establish priority was not unusual.  Among others, including Galileo, Huygens had made a similar obscure announcement to avoid oversharing of details.

Battling with fellow scientists was not out of the ordinary for Hooke.  His feud with Sir Isaac Newton ran so deep that following Hooke’s death, Newton, having freshly ascending to President of the Royal Society, destroyed all portraits of Hooke.  Newton would follow Hooke’s law of elasticity with his own observations on viscous bodies, which, in a somewhat symmetric fashion, exhibit a linear relationship between force and velocity.  The force to shear a viscous fluid is proportional to its velocity.  In other words, to double the velocity of a fluid confined between two plates would require twice the force.  The magnitude change in force required is the viscosity, which is appropriately the resistance of a fluid to deformation.  For elastic bodies, the elastic modulus is the value that describes the relationship between force and extension (put more directly, the elastic modulus is the ratio of stress to strain – i.e., force to displacement – at very small displacement).  The restriction to small displacements is because Hooke’s linear relationship fails as a material starts to yield or otherwise plastically deform.  As materials start to stretch, the additional force required to continue stretching generally decreases (“strain softening”).  Some materials can actually become stiffer during stretching, which is known as strain hardening.  Skin is an interesting example – it is pretty easy to stretch your cheek a small amount, but to continue pulling it requires significantly more force (it is not advised to test this out too thoroughly).  Fluids can undergo analogous deviations from Newton’s law linearly relating force and velocity, but since these are generally reversible, we describe this behavior by changes in viscosity (changes in modulus aren’t usually reversible, so they are more difficult to describe).  If the viscosity decreases, the fluid is shear thinning (think ketchup – it only flows with force applied).  Shear thickening fluids show an unexpected increase in viscosity with force, which is usually due to a crowded packing of small particles.  People running on a fluid containing large amounts of corn starch is the classic example of this effect.

Illustration of Hooke’s law.  A vertically oriented spring is at rest on the left.  The spring extends in constant steps as units of weight are added.  This simple observation is observed in all solid bodies for relatively small extensions.

Illustration of Hooke’s law.  A vertically oriented spring is at rest on the left.  The spring extends in constant steps as units of weight are added.  This simple observation is observed in all solid bodies for relatively small extensions.

It is worth including a few final notes to better appreciate the contributions of Hooke.  He made several significant observations on gravity, notably that gravity is an attractive force, that bodies tend to move in straight lines unless deflected by another force, and that the force of gravity increases as the distance between bodies is reduced.  These claims are at the core of what we now call Newtonian physics because Isaac Newton was the first to prove them via the rather trivial task of inventing calculus.  Hooke separately made some significant contributions to timekeeping, notably through improvements to pendulums, invented a ratcheting system for raising anchors, proposed that accurate time keeping could be used to determine longitude, and made developments in the design and use of microscopes of telescopes.  His microscope studies led to his suggestion that some species of the past may no longer be alive today (extinction), which offended the establishment of the time as theologically unacceptable.  He made a number of contributions to the determination of stellar distance, analysis of lunar craters, planetary systems, and stars, and in spare time, proposed a mechanistic model of human memory, served as Surveyor of the City of London, worked as an architect, and proposed a redesign of the street system on a grid structure.

Image credits:

The Monument illustration, Public Domain

Hooke inscription at The Monument, Wikipedia

Spring illustration, Hooke’s De potential restitutive frontispiece.

Hooke’s law illustration, SchoolPhysics