Electrical Instruments

IDENTIFICATION OF ELECTRICAL SCHEMATIC SYMBOLS
Circuit layouts and schematic diagrams are a simple and effective way of showing
pictorially the electrical connections, components and operation of a particular electrical circuit
or system. Basic electrical and electronic graphical symbols called Schematic Symbols are
commonly used within circuit diagrams, schematics and computer aided drawing packages to
identify the position of individual components and elements within a circuit.
Graphical symbols not only identify a components position but the type of electrical
element too, whether it is resistive, inductive, capacitive, mechanical, etc. Thus in circuit
diagrams and schematics, graphical symbols identify and represent electrical and electronic
devices and show how they are electrically connected together while drawing lines between
them represents the wires or component leads.
STUDY OF ELECTRICAL MEASURING INSTRUMENTS
Our life is incomplete without electricity. We need electricity for every purpose, be it
an entertainment (listening to music or watching television), working on a computer, using
washing machines or air conditioners. Everything needs electricity to operate. However, we
need to ensure whether everything is working appropriately and accurately. This necessitates
the importance of electrical measuring instruments.
Electrical measuring instruments play a huge role in a variety of things such as agriculture,
weather, automobile industry, health care centres and many more, in the modern society. Since they
have good potential to provide accurate results, we can perform our role with great reliance. Similarly,
when we want to verify the readings, we may require calibrating electrical measuring instrument to
match the readings of a particular standard.
Electrical measuring instruments as the name suggests are equipment used for gauging various
electrical aspects. These instruments measure the resistance or current, voltage, power, voltage
frequency, flux and so on. They can be classified into different types
Most commonly used electrical measuring instruments in electrical laboratories are,
 Ammeter
 Voltmeter
 Wattmeter
Ammeter
Ammeter (abbreviation of Ampere meter) is a measuring instrument used to measure
direct or alternating electric current in an electric circuit. Electric currents are measured in
Amperes (A), hence the name. The ammeter is usually connected in series with the circuit in
which the current is to be measured. An ammeter usually has low resistance so that it does not
cause a significant voltage drop in the circuit being measured.
Voltmeter
A voltmeter, also known as a voltage meter, is an instrument used for measuring the potential
difference, or voltage, between two points in an electrical or electronic circuit. Some voltmeters are
intended for use in direct current (DC) circuits; others are designed for alternating current (AC) circuits.
Specialized voltmeters can measure radio frequency (RF) voltage.
Wattmeter
A wattmeter is an instrument which is used to measure electric power given to or developed
by an electrical circuit. Generally, a wattmeter consists of a current coil and a potential coil.
When the wattmeter is connected in a circuit to measure power, the current coil carries the load
current and potential coil carries a current proportional to the load voltage.
CLASSIFICATION OF MEASURING INSTRUMENT
1. Absolute Instrument
The absolute instrument gives the value of measures quantities regarding the physical
constant. The physical constant means the angle of deflection, degree and meter constant. The
mathematical calculation requires for knowing the value of a physical constant. The tangent
galvanometer is the examples of the absolute instruments.
2. Secondary Instrument
In the secondary instrument, the deflection shows the magnitude of the measurable
quantities. The calibration of the instruments with the standard instrument is essential for the
measurement. The output of this type of device is directly obtained, and no mathematical
calculation requires for knowing their value.
Digital Instrument: The digital instrument gives the output in the numeric form. The
instrument is more accurate as compared to the analogue instrument because no human error
occurs in the reading.
Analog instrument: The instrument whose output varies continuously is known as the
analogue instrument. The analogue instrument has the pointer which shows the magnitude of
the measurable quantities.
3. Deflection Type Instrument

The instrument in which the value of measuring quantity is determined through the
deflection of the pointer is known as the deflection type instrument.
The deflection type instrument is further sub-classified into three types.
1. Indicating Instrument – The instrument which indicates the magnitude of the measured
quantity is known as the indicating instrument
2. Integrating Instrument - – The instrument which measures the total energy supplied at
a particular interval of time is known as the integrating instrument. The total energy measured
by the instrument is the product of the time and the measures electrical quantities.
The energy meter and watt-hour meter are the examples of integrating instrument.
3. Recording Instrument – The instrument records the circuit condition at a particular
interval of time is known as the recording instrument. The moving system of the recording
instrument carries a pen which lightly touches on the paper sheet. The movement of the coil is
traced on the paper sheet. The curve drawn on the paper shows the variation in the measurement
of the electrical quantities.
Instruments used for measuring Different type of electric supply
1) Moving Iron Type (MI) – both for AC and DC

2) Permanent magnet Moving coil type (PMMC) – For DC measurement
3) Induction type instrument – For AC measurement only
4) Hotwire instrument – used for measurement of both AC and DC
5) Dynamo meter type instruments – for both AC and DC measurement
6) Electrostatic instruments - for both AC and DC measurement

Electrical Instruments

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IDENTIFICATION OF ELECTRICAL SCHEMATIC SYMBOLS

Most commonly used electrical measuring instruments in electrical laboratories are,

CLASSIFICATION OF MEASURING INSTRUMENT

3. Deflection Type Instrument

Instruments used for measuring Different type of electric supply

1) Moving Iron Type (MI) – both for AC and DC

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