2 Length The new definition of the meter was adopted in 1983 and the metre was defined as the distance that light travels in a vacuum in 1/299792458 seconds. Fractions and multiples of the metre are given below: 1 nanometre = 1 nm = 10−9m (a few times the size of the largest atom) 1 micrometre =1µm = 10−6m 1 millimetre = 1 mm = 10−3m 1 centimetre = 1 cm = 10−2m 1 kilometre = 1 km = 103m (size of some bacteria and living cells) (diameter of the point of a ballpoint pen) (diameter of the little finger) (a 10-minute walk) The following instruments are used to measure length: 1. Metre rule: The metre rule is often used to measure distances of a few centimetres to some metres, for example, the dimensions of a table or a room. When longer distances are involved, a tape rule can be used. Both the metre rule and the tape rule are graduated in centimetres and millimetres. 2. Vernier callipers: When measurements are required to accuracy greater than 1mm, the Vernier scale may be used. Thus Vernier callipers are used in the measurement of the inner and outer 2
