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Systems of Measurements & Measuring Devices

“When you can measure what you are speaking about and express it in numbers, you know something about it; but when you cannot express it in numbers your knowledge is of meager and unsatisfactory kind" — Lord Kelvin

In science, measurement of various quantities is an act of immense importance. Measurement provides a firm ground of belief as to a scientific proposition. Any scientific proposition may be looked upon as a fiction if it is not expressed in a mathematical form; it gives one-a superficial idea.

Physical quantity

A physical quantity is one which is related to a material body or an event and can be measured in terms of a universally accepted standard quantity of its kind. Example: The book is not a physical quantity, but its length, volume, density, etc. are the physical quantities. Physical quantities are of two types. They are scalar and vector. A scalar physical quantity can be expressed with its magnitude but without any direction. Ex - mass, length, temperature, works etc. A vector quantity can be expressed with its magnitude and direction. Ex - velocity, displacement, force, weight etc.

Mass, length and time are considered as the fundamental quantities. Each of these can be measured independently. In system, there are seven fundamental quantities. They are length, mass, time, electric current strength, temperature, quantity of matter and luminous intensity of light. Derived physical quantities are those, each of which associates one or more fundamental physical quantities.

Unit

Unit is the universally accepted definite amount of a physical quantity taken as a standard for the measurement of the same physical quantity of any amount. Ex – gram (g), meter (m), second (s) etc.

Some physical quantities have no units, since each is expressed by a ratio of similar physical quantities.

Ex – mechanical advantage, velocity ratio, refractive index, atomic weight etc.

Measuring devices

a.	To measure length, a linear scale rod also called a ruler is generally used.
b.	For measuring volume of liquid a measuring cylinder is used.
c.	To determine the weight of a body a spring balance is used.
d.	To measure time wall clock, wrist watch, stop watch etc are used. Stop watch is used to note time of an event of short duration.