From: Douglas Jensen Sorry - my Metrology brain kicked in - and since we are having so much fun, I decided to chime in with the definitive answer on two definitions: The Meter and The Second. I don't claim to know all this stuff, but I do know where to find it: http://physics.nist.gov/cuu/Units The origins of the meter go back to the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. The astronomer Christian Huygens suggested defining the meter as the length of a pendulum having a period of one second; others suggested defining the meter as one ten-millionth of the length of the earth's meridian along a quadrant (one fourth the circumference of the earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum. Thus, the meter was intended to equal 10-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator. However, the first prototype as short by 0.2 millimeters because researchers miscalculated the flattening of the earth due to its rotation. Still this length became the standard. (The engraving at the right shows the casting of the platinum-iridium alloy called the "1874 Alloy.") In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice. In 1927, the meter was more precisely defined as the distance, at 0�, between the axes of the two central lines marked on the bar of platinum-iridium kept at the BIPM, and declared Prototype of the meter by the 1st CGPM, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other. The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. (Writers comments: Actually the number I remember was 299792457.4 m/s, but I seem to recall that being updated when something called into account the conditions under which that was measured. Now however, we have the speed of light defining the meter as in): Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m�s-1. The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under the conditions specified in 1889. The current definition of the second is duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. Now if I can only find my damn hyperfine level transition detector here somewhere..... DJ Jon Elson <[EMAIL PROTECTED]> on 03/05/2001 06:08:52 PM Please respond to [EMAIL PROTECTED] To: Multiple recipients of list proteledausers <[EMAIL PROTECTED]> cc: (bcc: Douglas Jensen/486394/US/EKC) Subject: Re: [PROTEL EDA USERS]: Give up, there is no point (Ex: Confused Newbie on Footprints)
Ian Wilson wrote: > On 10:10 AM 6/03/2001 +1100, John Haddy said: > >By definition, 1 inch = 25.4mm exactly. This was standardised > >some decades ago. > Off topic a little... > > But the metre (and hence inch) are derived quantities (variables) so all > our footprints are totally useless! Do you hear - *useless*! > > The speed of light, c, is constant. Any improvements in the how fast light > travels does not affect the number in metres per second. The length of the > metre is changed instead. So there is no point in trying to design > footprints as the next time a physicist improves the estimate of c we have > to go back and update all out libraries. This is actually not true. The Metre is based on an exact number of wavelengths of the Neon emission line that is characteristic of the HeNe laser, somewhere around 637 nM, I think. So, the length of objects is not changed by speed of light measurements. And, the wavelength of the Neon atom will remain quite stable when measured properly. Jon * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * This message sent by: PROTEL EDA USERS MAILING LIST * * Use the "reply" command in your email program to * respond to this message. * * To unsubscribe from this mailing list use the form at * the Association web site. You will need to give the same * email address you originally used to subscribe (do not * give an alias unless it was used to subscribe). * * Visit http://www.techservinc.com/protelusers/subscrib.html * to unsubscribe or to subscribe a new email address. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
