Submitted on: 26 February 2010.
Revised on: 02 June 2010.
Accepted on: 11 July 2010.
___________________AMERICAN JOURNAL OF PHYSICS---AIP PUBLISHING
Submitted on: 19 July 2006.
Revised on: 08 November 2006.
Accepted on: 26 November 2006.
___________________AMERICAN JOURNAL OF PHYSICS---AIP PUBLISHING
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Submitted on: 11 May 1998.
Revised on: 11 August 1998.
Accepted on: 03 September 1998.
___________________AMERICAN JOURNAL OF PHYSICS---AIP PUBLISHING