# MEASUREMENT OF LENGTH

When we measure the distance between any two points in term of width, thickness, depth, and height, we actually carry out the measurement of length.

Definition of length –

Length is the measure of distance between any two points.

To understand the importance of measurement consider this – if you have two pencils and want to know which one of the two is longer, then you can easily find it out by placing both of them side by side. However, there are situations when you cannot move objects and place them side by side. Take the example of your’s teacher table and the dining table at your home. If you want to know which one of the two is longer, you just cannot carry teacher’s table to your house or your dining table to school. However, if you measure the length of teacher’s table by the span of your palm and compare it with the length of the dining table by the span of your palm, you will easily find out the which one of the two tables is longer.

Courtesy: Goyal Brothers Prakashan

Now think what did you did to find the answer. You did not move any of the tables. Instead, you moved from the school to the dining room. To make the measurement, you used the third object, that is your palm. Similarly, various other instruments used for carrying out measurement of length are a ruler, meter scale, measuring tape, vernier caliper, and screw gauge.

##### STANDARD UNIT FOR MEASUREMENT OF LENGTH

In the above example, you used the span of your palm or hand to measure length. Although this method of carrying out measurement of length may be simple it is not accurate. For example, if you and your father both measure the length of the sofa in your drawing room with a hand span. The number of hand span measured by you on the same sofa will be more than what your father measured. This is because your hand span is smaller than your father.

So, to maintain uniformity, people all over the world use * meter* as the standard unit for length. To ensure that it is the same meter used everywhere, the standard meter was defined by scientists as below:

Definitions of the meter (1798) –

One meter is one ten-millionth (1/10,000,000) of the distance from the Equator to the North pole along the longitude passing through the Paris in France.

Definitions of the meter (1927) –

Meter is the length between two marks on a metal bar made of 90% platinum and 10% iridium alloy. This bar is maintained at constant temperature of 0

^{o}C and is placed on two rollers. It is kept near Paris in France.

Definition of the meter (1988) –

Length of the path travelled by light in a vacuum in 1/299,792,458 of a second.

To know more about the history of the meter, visit: History_of_the_metre

##### MULTIPLES AND SUB-MULTIPLES OF LENGTH

The unit meter is too small for measuring some lengths and too large for others. So, it was decided to increase it in multiple of 10 for large measures and decrease it by sub-multiples of 10 for small measures. These multiples and sub-multiples of the meter are given special names as given below:

**Multiples of Meter**

Decameter (dam), 1 dam = 10 meters = 10 m

Hectometer (hm), 1 hm = 100 meters = 100 m

Kilometer (km), 1 km = 1000 meters = 1000 m

Astronomical Unit (AU), 1 AU = 149,600,000,000 meters = 1496 x 10^{8} m

Light year (ly), 1 ly = 9,561,000,000,000,000 meters = 9461 x 10^{12} m

We use the astronomical unit and the light year to measure a distance between extremely large distances, such as distances between planets and galaxies in the Universe.

**Sub-multiples of Meter**

Decimeter (dm), 1 dm = 0.1 meters = 1.0 x 10^{-1} m **or** 1 m = 10 dm

Centimeter (cm), 1 cm = 0.01 meters = 1.0 x 10^{-2} m **or** 1 m = 100 cm

Millimeter (mm), 1 mm = 0.001 meters = 1.0 x 10^{-3} m **or** 1 m = 1,000 mm

Micrometer (μm), 1 μm = 0.000001 meters = 1.0 x 10^{-6} m **or** 1 m = 1,000,000 μm

Nanometer (nm), 1 nm = 0.000000001 meters = 1.0 x 10^{-9} m **or** 1 m = 1,000,000,000 nm

Angstrom (Å), 1 Å = 0.0000000001 meters = 1.0 x 10^{-10} m **or** 1 m = 10,000,000,000 Å

We use units like the Nanometer and the Angstrom for the measurement of extremely small sizes, such as the diameter of atoms or molecules.

##### OTHER UNITS FOR MEASUREMENT OF LENGTH

Apart from the units mentioned above, some people use foot, inch, yard and mile as units for measurement of length.

Inch (in), 1 in = 2.54 cm

Foot (ft), 1 ft = 12 in or 30.48 cm

Yard (yd), 1 yd = 3 ft or 36 in or 91.44 cm

Mile (mi), 1 mi = 1760 yd or 5280 ft or 63,360 in

##### INSTRUMENTS USED FOR MEASUREMENT OF LENGTH

In the early days, people used to measure length with the help of body parts, such as outstretched palm, foot, arm length (cubit). But you can see that such instruments cannot serve as the standard for measuring length. The simple reason is that they are going to be different for different people. So, in order to avoid confusion scientists developed standard instruments for carrying out a measurement. Standard instruments are those which are acceptable to the majority of the people as a basic instrument for carrying out the measurement. Some of the standard instruments used for the measurement of length are a ruler, meter scale, measuring tape, vernier caliper, and screw gauge.

To measure very long length like the length of a road we use measuring tape. For measuring very small lengths like the thickness of nail or paper, we use vernier caliper and screw gauge. For other common measurements of day to day household objects, we use rule and meter scale.