Methods of measurement

To understand any simple concept, we need to understand the fundamental definition of it. In the world of physics, every physical quantity is compared with its standard unit and that can be defined as measurement. So that brings us to understand what a standard unit is! Any unit which can be easily reproduced and also accepted internationally. Methods of measurements are used to measure these physical quantities which are independent of each other. These are fundamental quantities and their units are fundamental units. The system which we follow currently is called SI System and it consists of 7 fundamental units. Let us understand these in detail!

Fundamental units:

As said in the introduction, there are 7 fundamental units, along with them there are 2 supplementary units as well.

Fundamental Quantities	Fundamental units
Length	meter(m)
Mass	kilogram(kg)
Time	second(s)
Temperature	kelvin(K)
Electric current	ampere(A)
Luminous intensity	candela(cd)
Amount of substance	mole(mol)

The supplementary fundamental units are used to measure plane angle and solid angle and the units are called radian and steradian respectively.

Plane angle	Radian(rad)
Solid angle	Steradian(sr)

System of units:

Initially there were 3 different systems used in different countries. These systems were CGS, FPS and MKS systems. But this was creating confusion for all the scientists in the world, so they decided to adopt an international system of units which we use currently called the SI system of units.

• CGS : The abbreviation stands for centimeter,gram and second and are used to express the length,mass and time respectively
• FPS : The abbreviation stands for foot,pound and second and are used to express the length,mass and time respectively
• MKS : The abbreviation stands for meter, kilogram and second and are used to express the length,mass and time respectively

Other important units of length:

All the units which we came across now, are usually used to measure small distances. For measuring large distances such as planets,stars and galaxies we need some bigger units of length that is, ‘astronomical unit’, ‘light year’, ‘parsec’ etc.

• An astronomical unit can be defined as the average distance of separation between the sun and the Earth,

1 AU = 1.496×10¹¹m

• The amount of distance traveled by light in the vacuum in a period of one year is called light year,

1 light year = 9.46×1015</sup? m

• The amount of distance at which an arc of the length of an astronomical units exhibits an angle of one second at a point is called parsec,

1 parsec = 3.08×10¹⁶ m

Different methods of measurement:

There are a various number of methods through which a physical variable can be measured and the selection of the method is done according to the accuracy required. Few of the methods are,

• Direct method
• Indirect method
• Comparative method
• Substitution method

Direct method:

As the name itself indicates, in this method the quantity’s value is obtained directly by comparing the unknown quantity with a standard one. This is a simple method where no complex mathematical calculations are involved. Examples : Measuring the length of the cloth using a standard cloth meter, measuring the temperature using a thermometer.

Indirect method:

Whenever direct methods cannot be used, we use this method to obtain the quantity’s value. In indirect measurement the quantity which has to be measured is determined using a mathematical relationship. Example : We know that Volume = Mass/Density Using this, we can measure the volume by first measuring the mass and the density.

Comparative method:

In this method, the value which is already known is compared with the quantity which has to be measured. Example: Comparison of the sound level metre with reference to the sound level metre

Substitution method:

This method is widely used in the field of engineering and technology. It requires the replacement of a value with the quantity which has to be measured with the known value of the same quantity. To explain it in an easier way, whichever quantity has to be measured is done using the direct method on an indicating machine by the replacement of the measuring quantity with another known quantity that produces the similar effect on the indicating machine. Example : Calibration of weights obtained in a laboratory using a substitution method.

Dimensions:

• The power to which the fundamental units which are mass, length and time are raised to represent the given fundamental quantity is defined as the Dimension of a physical quantity
• These are used in checking the results, conversion of units from one system to another, deriving relationships in between different physical quantities

Errors:

• The true value of any physical quantity is generally different from the measured value, uncertainty is common and this uncertainty is called an error

Different causes of an error: Least count error, Instrumental error, Random error, Accidental error etc

Conclusion

The system of units is widely used and accepted internationally, this system is called the SI system which consists of 7 fundamental units and 2 supplementary units. All the physical quantities are derived from these. Measurement plays a vital role in our life and there are a lot of methods which we can use, including direct method, indirect method, comparative method and substitution method. Dimensions are the power to which the fundamental units which are mass,length and time are raised to represent the given fundamental quantity. Errors are uncertainty which can be seen in the measurement of the physical quantities.