## >> Tuesday, October 4, 2011

All measurements in physics, even of such things as electric current, are related to the three chosen fundamental quantities of length, mass and time. Until about the year 1800, workers in various countries used different systems of units. Thus, while the English used inches, a continental scientist would measure lengths in centimeters. Fortunately, this unsatisfactory situation has now been changed by the efforts of various international committees of scientists who have met for discussion regularly over many years.
In 1960 the General Conference of Weights and Measures recommended that everyone should use a metric system of measurement called the International System of Units (abbreviated SI in all languages). The SI units are derived from the earlier MKS system, so called because its first three basic units are the meter (m), the kilogram (kg), and the second (s).

Measurement of length
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 Measures at Sevres near Paris. Copies of this standard were sent to other countries. Now the trouble with metal standards of this kind is that they are liable to undergo minute changes in length as the years go by. For instance, tests have shown that the imperial standard yard has shrunk by a few parts in a million since it was made in 1845. Small though this error is the exacting requirements of modern science demand something better. The standard meter is, of course, open to the same objection. In 1983 the General Conference of Weights and Measures redefined the meter as the length of the path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second.

For most practical purposes we still have to use metal standards which are checked by an interference comparator and this uses the wavelength of light. However some simple experiments for measuring the wavelength of light.
Various other metric units of length are related to the meter by either multiples or sub multiples of 10. 1 kilometer (km) = 1000 meters (m)
1 meter (m) = 100 centimeters (cm)
1 centimeter (cm) = 10 millimeters mm)
Very small lengths are measured in micrometers (.tm) and nanometers (nm).
1 meter = 1 000 000 (or l06)?m
= 1 000 000 000 (or 109)nm
For day-to-day work in elementary physics laboratories we use meter and half-meter rules made of boxwood. They are graduated in centimeters and millimeters. Care should he taken to avoid damage to the ends of these rules, as they do not have a short un-graduated portion at the ends to take the wear. 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 millimeter scale giving a reading of 28 cm 3 mm. Since, in science, we invariably use decimals rather than fractions, we write this as 28:35 cm.