Top Eleven: Henry Cavendish

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.

When: 1797.

What: Cavendish’s modern claim to fame is the torsion pendulum experiment, an idea that originated with John Michell, who died before completing it.

i-74a0221306e16abb568132ab077affa0-Cavendish1.gifThe 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.

i-642d88b3315f2f15218358f75b2c6298-sm_Gfig3.jpgMore 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…”

Comments

  1. #1 Trent Goulding
    January 27, 2006

    I probably either missed something in the explanation, or the answer is “intuitively obvious to the meanest understanding,” so feel free to rub my nose in it, but how did he actually measure the amount of twist on the wire? What mechanism, especially given early nineteenth century tech, can measure such a small amount of twist? Not saying he didn’t do it, obviously he did, I’m just curious how…

  2. #2 Chad Orzel
    January 27, 2006

    I probably either missed something in the explanation, or the answer is “intuitively obvious to the meanest understanding,” so feel free to rub my nose in it, but how did he actually measure the amount of twist on the wire? What mechanism, especially given early nineteenth century tech, can measure such a small amount of twist? Not saying he didn’t do it, obviously he did, I’m just curious how…

    The wire twists through a very small angle, but the masses out at the ends of the dumbbell can move a measurable distance. I assume he either measured the displacement of the masses at the ends, or possibly a pointer extending out from the end of the dumbbell.

    Modern versions of the experiment usually involve attaching a mirror to the wire, and bouncing a laser off it. Then, you monitor the position of the laser spot on a wall several meters away.

  3. #3 GP1
    December 9, 2006

    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.

    I tend to believe that dead cannot make measurements of scientific grade. When G was invented by the hardcore British Newtonists in the 19th century Cavendish was already dead. How do you think he managed to measure the so-called constant G he did not know about?

  4. #4 Peter Duveen
    April 14, 2007

    The comments posted have been most instructive, and suggest that the author of the above article on Cavendish was not privy to Cavendish’s original papers on the subject. This is a glaring deficiency, but one that is so commom that it is generally glossed over altogether in the educational establishment. This is because we have an educational system that more often than not lacks intellectual honesty. However, at least the author wrote something on the subject, and this, in conjunction with the other interesting comments, can be the beginning of an important discussion. My understanding is that Cavendish did not express his results in the form of a universal constant, but rather by a measurement of the specific gravity of the earth.

  5. #5 Peter Duveen
    June 14, 2007

    Would you please remove my former comment. On reviewing it, I found that I was inappropriately critical of this gentleman’s efforts to educate the public. Please contact me at my email address if you need to verify that I am indeed the author of this comment.

  6. #6 por
    January 7, 2009

    i study about science but i just read “A Short History of Nearly Everthing” which i bought for own new year present, Henry Cavendish is one of the most scienctist in this book that great!. he found many many thing, but i want to known and saw the mechanical that created by John Michell, and found it in this site. wow wow be cool!!!!!

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