Click here to purchase the entire book in PDF format. The metre In the 1790's the French Academy of Sciences decided that they would define a unit of length called a mètre (from the Greek word metron meaning ``a measure'') as one ten-millionth of the distance from the equator to the North Pole on a meridian (a line running North-South on the Earth) that goes through Paris. In 1795, they produced a prototype ``metre bar.'' This was an actual bar of brass that was officially 1 metre long. The final version was made of platinum and was made in 17 99 and put in the French National Archives. The official definition of the meter changed a number of times since the 1700's. The version we use today was defined on October 21, 1983 at the seventeenth (CGPM). It states that 1 metre is ``the length of the path travelled by light in vacuum during a time interval of 1 / 299,792,458 of a second.'' (www.bipm.fr) This definition is interesting (depending on what you consider to be interesting) because it makes the speed of light a nice number... exactly 299,792,458 m/s. Another interesting point is that one of the original definitions of the metre that was being considered was the length that you had to make a pendulum to make its travelling time 1 second. If you don't believe this one, take a piece of string 1 m long and tie something heavy to one end. Tie the other end to something that doesn't move and time how long it takes to swing back and forth. If it's shorter than a metre, then it will take less than a second, longer than a metre will result in it taking longer. (Also note that it doesn't matter how heavy the weight is... Galileo Galilei proved that one to us. )