Tuesday, February 26, 2013

Compton's Water Based Foucault Pendulum

A further review of the article[1] I mentioned yesterday, (Dr. Tatum on the Coriolis effect as a navigational aid for birds), turned up a very interesting reference to Arthur Compton of Compton scattering fame (picture 1)!

It turns out that Compton built an apparatus in 1915 at Wooster University, (now Wooster College) that accurately measured the rotation speed of the Earth allowing Compton to determine the length of a day, his latitude and longitude all without any astronomical references.  It was a sort of Foucault pendulum constructed with a tube of water.

To read all about Compton's apparatus and method in Scientific American, go to [5]:

or in the blog version of this post, just scroll to the bottom of the post for an embedded version of the article.

Picture 2, below, shows a simplified version of Compton's apparatus sitting directly on the axis of rotation of the Earth.  Here's the basic idea.  The circular tube is filled with water and the tube is rotating along with the Earth.  After some time, the rotational speed of the water will match the rotational speed of the tube and there will be no relative motion between the two.  At this point, flip the tube over 180 degrees.  The water will tend to keep rotating in the direction it was going in while the tube will now be moving in the opposite direction.  By measuring the speed of the water, you get a measure of the Earth's rotational speed.

Big deal you say.  A Foucault pendulum can give the same results you say.  Well, as it turns out, it has a few drawbacks with respect to Compton's apparatus.  Consider the case where either apparatus wasn't sitting at the Earth's pole.  Then, you only get a measure of the Earth's rotation with respect to the axis of rotation perpendicular to the horizontal plane at your location.  With a pendulum, you're done.  You can show that the Earth  is rotating, and knowing your location on the Earth, and making an assumption about the Earth's axis of rotation, you can determine how quickly the Earth is rotating.  The annoying part, however, is making that assumption about the Earth's axis of rotation.  With Compton's apparatus, you simply make two more measurements.  Your second measurement is made with the plane of the ring perpendicular to the horizontal plane at your location.  Then, you merely rotate the tube 90 degrees while still keeping it's plane perpendicular to horizontal.  You now have three perpendicular components of the Earth's rotational speed and can re-construct both the Earth's total rotational speed and the direction of the axis of location.  Compton's results are reported in the Scientific American article and shown below in picture  3:

Compton's article in Scientific American:

1.  Tatum's navigation article

2.  +American Physical Society's Physical Review article on Compton's apparatus
DOI: 10.1103/PhysRev.5.109

3.  Science Magazine on Compton's apparatus

4.  Compton biography

5.  Free Scientific American article

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