Measurement of Length

Category: Written by Nerella Bharat / 6:11 AM
We learn many things by careful observations of phenomena of nature. The ultimate test
of physical theory is its agreement with observations and measurement of physical
Physics is basically a science of measurements.
According to LORD KELVIN (1824-1907), one of the greatest scientists, unless you can
measure what you are speaking about and express it in numbers you have not started exact
For the measurement of any physical quantity we choose a constant quantity as a
standard. We compare this standard quantity with the given physical quantity to find the
number, which express how many times a standard quantity is contained in a physical
The standard quantity (Constant quantity), used for comparison, is called unit.
MEASUREMENT is the comparison of an unknown quantity with a known standard
quantity (Constant Quantity) or unit.
In order to express the result of measurement, we must know:
i) The unit in which a quantity is measured
ii) The numerical value which express how many times a standard quantity or unit is 
contained in a given physical quantity.

Example: If we have to measure weight of a book, we use standard weighing (kg) machine.
ON measurement, if we find that 5 times the standard weight, then the weight is 5 kg. In the
above statement 5 is the unit of physical quantity.
Magnitude of a Physical Quantity: The number of times a standard quantity is present in
a given physical quantity is called magnitude of physical quantity.
Physical quantity = (Magnitude) × (Unit)

Need for the introduction of standard Quantity:  From ancient times, man has been
facing the necessity of measuring and estimating various quantities. For measuring mass,
he used various kinds of standards. For measuring length, he used foot length or arm length
etc. However, pants of human body differ in lengths for different people. This lead to a lot of
However, with development in science, same standards of measurement adopted world
wide. This led to avoid all kinds of confusion.
The first serious attempt to standardise the units was made in 1889 in paris. However
some anomalies were left. In october 1960, another conference was held and the standard
units were revised.

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