Next up in the Top Eleven is an experiment whose basic technique is still in use today.
Who: Henry Cavendish (1731-1810), a British scientist who made a number of discoveries in physics and chemistry, but received credit for very few of them.
What: Cavendish’s modern claim to fame is the torsion pendulum experiment, an idea that originated with John Michell, who died before completing it.
The apparatus for the famous experiment, shown at left, consists of a dumbell-shaped pendulum hung from a very fine wire. Two larger masses (Cavendish used 350 lb lead spheres) are brought near the ends of the dumbell, where they exert a graviational force on the ends of the dumbell, causing it to twist slightly. The wire resists the twist, and after a little while, the system settles into a new position where the torque cause by the gravitational force is exactly balanced by the restoring torque from the twisted wire. By measuring the amount of twist, you can determine the strnegth of the force of gravity, and if you know the masses involved and the distance between them, you can determine the value of Newton’s graviational constant G.
Why It’s Important: Cavendish’s measurement of G was one of the first measurements of a universal physical constant, and allowed the first determination of the mass of the Earth. This, in turn, allowed people to determine the masses of all the other planets.
His experiment is also remarkable for being a ridiculously exacting measurement. The forces involved are so tiny that slight air currents are enough to disturb the system, so Cavendish actually set it up in a sealed room, and recorded the twist of the wire from outside with a telescope. His measurements were within 1% or so of the current value of G, and weren’t improved upon for something like a hundred years.
More importantly, the torsion pendulum technique is still in use today, and not only for torturing undergraduate physics students. The current best measurement of G, by the Eot-Wash group at the University of Washington uses an apparatus (pictured at left, click for a larger image, pictures stolen from their site) that is, at its heart, still a torsion pendulum. They also use variants of the basic idea for a bunch of other experiments, some of which have the potential to provide experimental tests for string theory.
Reasons to Vote for Him:: He pioneered a ridiculously precise technique for measuring tiny forces, and really set the standard for precision tests of gravity.
Reasons to Vote Against Him: “I hated that experiment when we did it in my college class…”