Fig. 1.1 gives a glimpse of some landmarks in the history of length measurement. The SI unit of length is the meter. originally defined as the distance. at 0 C between two lines on a platinum-iridium bar kept at the International Office of Weights and Me’ sures at Sevres near Paris. Copies of this standard were sent to other countries. ow the trouble with metal standards of this kind is that they arc liable to undergo ~int~ changes in length as the~ ye~rs go b~Fr instance. tests have show. that the imperial standard yard shown In Fig. 1.1 Has trunk by a few parts In a million Since it was made in 1845) Small though this error s, the exacting requirements of modern science deP.’and something better. The standard meter is. of course. open to the same objection.\!n 1983 the General Conference of Weights and Measures redefined the metro as the length of the traveled by light in a vacuum during a time interval of IIJ!9 792 458 of a scone For most practical spores we still have to use metal standards which are checked by an interference comparative (see page 5) and this uses the wavelength of light. We cannot go into details of this nor is it necessary to memories the above definition. However some seem IE experiments for measuring the wavelength of light will be described in chapter 26. carious other metric units of length are related to the meter by either multiples or multiple of 10.

Thus,

1 kilometer (km) = 1000 meters (m)
1 meter (m) = 100 centimeters (em)
1 centimeter (earn) = 10 millimeters (mm)
wry mall lengths are measured in micrometers (1) and metronomes (non).
I meter = I 000 000 (or ION)ll
= 1000 000 000 (or 109 ) NM
For day-to-day work in elementary physics laboratories we use meter and halftime rules made of boxwood. They are graduated in centimeters and millimeters. Care should be taken to avoid damage to the ends of these rules, as they do not have a hot undergraduate portion at the ends to take the wear. However, since a small

amount of wear is almost inevitable, it is best, whenever possible, to measure from the 10 em graduation and subtract 10 em from the reading at the other end. Owing to the thickness of the wood, the eye must always be placed vertically above the mark being read, in order to avoid errors due to parallax (see page 228)

We invariably use decimals rather than vulgar fractions, we write this as 28.35 cm.
The last digit has to be estimated.