The measuring instruments are a devices which is use  for measuring a physical quantity.  In Fact measurement is the process by which one can convert physical Parameters to meaningful number. The measuring process is one in which the property of an object or system under consideration is compared to an accepted standard unit, a standard defined for particular property.  In the physical sciences, quality assurance, and engineering,  measurement is the activity of obtaining and comparing physical quantities of real-world objects and events.


The Method of measurement may be broadly classified into two categories.

Direct methods: In this method, the  unknown quantity is directly compare against a standard quantity and result is obtain as a numerical number and a unit. Direct method is common for measurement of physical quantities like mass length and time.

Indirect methods: Measurements by direct methods are not always possible, feasible and practicable because some time human factor is involve and inaccuracy. in engineering applications measurement system are use which require need of indirect method for measurement purpose.


Basically there are three types of measuring instruments and they are
  1. Electrical measuring instruments
  2. Mechanical measuring instruments.
  3. Electronic measuring instruments.

Here we are interested in electrical measuring instruments so we will discuss about them in detail. Electrical instruments measure the various electrical quantities like current, voltage, power factor,and power. electrically system normally depend upon a mechanical meter movement as indicating device.the mechanical movement has some inertia and therefor these instrument have limited time response 


There are many way in which instruments can be classified. Broadly instruments are classified into two categories.

1. Absolute instruments 2. Secondary instruments.

 1. Absolute instruments: these instruments are give magnitude of measuring quantities in  term of physical constant of instrument. 

Example of this class instruments are Tangent Galvanometer and Rayleigh's current balance.    

 2. Secondary instrumentsThey are defined as the instruments that give a ready measure of the quantities with the help of graduated scales. The value of the electrical quantity to be measured is determined from the deflection of these instruments. With an absolute instrument these instruments are calibrated.

Example of this class instrument are  voltmeter,  glass thermometer and  pressure gauge.

Secondary instrument work on two mode Analog mode and Digital mode.Analog mode signal vary in continuous manner. the device which produce these signal called analog devices.signal which vary in discrete step. the device which produce these signals are called digital device.


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