Classification of instruments according to function:

1) Indicating instruments2) Recording instruments3) Integrating instruments

Explanation:

1) Indicating instruments: Indicating instruments indicate, generally the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc.

2) Recording instruments: These instruments record continuously the variation of any electrical quantity with respect to time. In principle, these are indicating instruments but so arranged that a permanent continuous record of the indication is made on a chart or dial. The recording is generally made by a pen on a graph paper which is rotated on a dice or drum at a uniform speed. The amount of the quantity at any time (instant) may be read from the traced chart. Any variation in the quantity with time is recorded by these instruments. Any electrical quantity like current, voltage, power etc., (which may be measured lay the indicating instruments) may be arranged to be recorded by a suitable recording mechanism.

3) Integrating instruments: These instruments record the consumption of the total quantity of electricity, energy etc., during a particular period of time. That is, these instruments totalize events over a specified period of time. No indication of the rate or variation or the amount at a particular instant are available from them. Some widely used integrating instruments are: Ampere-hour meter: kilowatthour (kWh) meter, kilovolt-ampere-hour

Comparison Analog and Digital Instruments:

Analog Digital

Analog signal is a continuous signal which represents physical measurements

Digital signals are discrete time signals generated by digital modulation

Display results in the form of wave signal Display results in the form of binary bitUses continuous range of values to represent information

Uses discrete or discontinuous values to represent information

Low cost and portable Cost is high and not easily portableAnalog instrument draws large power Digital instrument draws only

negligible powerAnalog instruments are rarely used Digital instruments are widely used

Impedance is low Impedance is highAnalog instruments usually have a scale which is cramped at lower end and give considerable observational errors

Digital instruments are free from observational errors like parallax and approximation errors

Deflecting torque:

The deflecting torque is produced by making one of the magnetic, heating, chemical, electrostatic and electromagnetic induction effects of current or voltage and cause the moving system of the instrument to move on a calibrated scale from its zero position to indicate on a graduated scale the value of electrical quantity being measured. The method of producing this torque depends upon the type of instrument.

Types of errors in measurements:

There are three basic types errors in measurements which given below:

1) Systematic Errors2) Random Errors3) Gross Errors

1) Systematic Errors: Systematic errors are those that remains constant with repeated measurements. Since they arise from inaccuracies in the manufacture of an instrument or from improper adjustment or application of an instrument. Some of the more common systematic errors are:

Zero Error: All readings are in error by the same amount. An example would be a voltmeter that reads 1 V low on all readings, including zero.

Scale Error: This depends on the magnitude of the reading. An example would be a voltmeter that reads 1 V high at 10 V, 2 V high at 20 V, and so on.

Response time error: This is due the instruments inability to follow dynamic changes in the measured quantity.

Loading error: The instrument extracts sufficient energy from the system under measurement so that the value of the measured parameter is changed.

2) Random Errors: Random errors are those errors due to unknown causes and are observed when the magnitude and polarity of a measurement fluctuate in an unpredictable manner. Some of the more common random errors are:Rounding error: This occurs when readings are between scale graduations and the reading is rounded up or down to the nearest graduation.Periodic error: This occurs when an analog meter reading swings or fluctuates about the correct reading.Noise: The sensitivity of the instrument is changed or the reading is altered by outside interference.Backlash: The reading either lags or leads the correct value because of mechanical play, friction or damping.Ambient influences: Due to conditions external to the measuring system, such as variation in temperature, humidity or atmospheric pressure.Random errors cannot normally be corrected but they can be minimized by a skilled observer.

3) Gross Errors: Gross errors, as the name implies are usually quite large and can be divided into two major categories:Human error: This occurs when the operator makes a mistake such as reading the wrong scale or reading.Equipment faults: This error source can be large and sometimes erratic.Gross errors cannot be eliminated but they can be significantly minimized by 1) careful operator attention and cross-checking of results, and 2) frequent equipment calibration.

Parts of CRT: A CRT is electronic tube design to display electrical data. The basic CRT consists of four major components1) Electron gun (2) Focusing and accelerating anodes (3) Horizontal and vertical deflection

plates (4) Evacuated Glass envelope (5) Base1) Electron gun: It is used for producing a strain of electrons.2) Focusing and accelerating anodes: These are used for producing a narrow and sharply focus

beam of electrons3) Horizontal and vertical deflection: These are used for controlling the path of the beam.4) Evacuated Glass envelope: With a phosphorescent screen which produces bright spot when

struck by a high velocity electron beam. The electron gun assembly consists of an indirectly heated cathode (K), a control grid (G), an accelerated anode A1, focusing anode A2 and accelerating anode A3.

5) Base: It provides connection to external circuitry

Construction and Working Principle of CRT:

The construction of CRT is shown below, the CRT uses electrostatic focus and electrostatic deflection, an arrangement found in most oscilloscopes.

Heater element is energized by alternating current to obtain high emission of electron from cathode control grid is bias negative with respect to cathode it controls the density of electron beam to focus the electron beam on the screen focusing anode is used. The focusing anode operate at a potential of twelve hundred volt ( 1200V) and accelerating anode at 2000 V to accelerate the electron beam.

Two pairs of deflection plates provided in the CRT these deflection plates are mounted at right angle to each other to provide electron beam deflection along vertical and horizontal axis of the screen. The screen consists of a glass which is coated by some florescent material lying zinc silicate. This is semitransparent phosphor substance. When high velocity electron beam struck the phosphorescent screen the light emits from it. The property of phosphor to emit light when its atoms are excited is called fluorescence.

Primary use of Kelvin Double Bridge:

The primary use of Kelvin Double Bridge is the precise measurement of low value resistance. The Kelvin double bridge is the modification of the Wheatstone bridge and provides greatly increased accuracy in measurement of low resistance.

Transducer: An electrical transducer is a device which is capable of converting the physical quantity into a proportional electrical quantity such as voltage or electric current. Hence it converts any quantity to be measured into usable electrical signal. This physical quantity which is to be measured can be pressure, level, temperature, displacement etc. The output which is obtained from the transducer is in the electrical form and is equivalent to the measured quantity. For example, a temperature transducer will convert temperature to an equivalent electrical potential. This output signal can be used to control the physical quantity or display it.

Types of Transducer: These are the followings:

Types of Transducer based on Quantity to be measured:• Temperature transducers (e.g. a thermocouple)

• Pressure transducers (e.g. a diaphragm)

• Displacement transducers (e.g. LVDT)

• Flow transducers

Types of Transducer based on the Principle of Operation• Photovoltaic (e.g. a solar cell)

• Piezoelectric Transducer

• Chemical Transducer

• Inductive Transducer

• Capacitive Transducer

• Electromagnetic Transducer

• Hall Effect Transducer

• Photoconductive Transducer

Types of Transducer based on Whether an External Power Source is required or not

Active Transducer

Passive Transducers

Light sensitive Transducer:

A Light sensitive Transducer is a passive devices that convert the light energy whether visible or in the infra-red parts of the spectrum into an electrical signal output. Light sensitive Transducer are more commonly known as “Photoelectric Devices” or “Photo Transducers” because they convert light energy (photons) into electricity (electrons).

Photoelectric devices can be grouped into two main categories, those which generate electricity when illuminated, such as Photo-voltaic or Photo-emissive etc., and those which change their electrical properties in some way such as Photo-resistors or Photo-conductors. This leads to the following classification of devices.

 Photo-emissive Cells – These are photo devices which release free electrons from a light sensitive

material such as cesium when struck by a photon of sufficient energy. The amount of energy the photons

have depends on the frequency of the light and the higher the frequency, the more energy the photons

have converting light energy into electrical energy.

• Photo-conductive Cells – These photo devices vary their electrical resistance when subjected to light.

Photoconductivity results from light hitting a semiconductor material which controls the current flow

through it. Thus, more light increase the current for a given applied voltage. The most common

photoconductive material is Cadmium Sulfide used in LDR photocells.

• Photo-voltaic Cells – These photo devices generate an emf in proportion to the radiant light energy

received and is similar in effect to photoconductivity. Light energy falls on to two semiconductor materials

sandwiched together creating a voltage of approximately 0.5V. The most common photovoltaic material

is Selenium used in solar cells.

• Photo-junction Devices – These photo devices are mainly true semiconductor devices such as the

photodiode or phototransistor which use light to control the flow of electrons and holes across their PN-

junction. Photo junction devices are specifically designed for detector application and light penetration

with their spectral response tuned to the wavelength of incident light.

Method of measuring unknown capacitance with help of bridge:

 Measurement of the unknown capacitance by Schering Bridge:The Schering bridge one of the most important ac bridge is used the extensively for themeasurement of capacitors.In the Schering bridge the arm 1 now contains a parallel combination of theResistor and the capacitor and standard arm contain only one capacitor. The standardCapacitor is usually a standard high quality mica capacitor.In the balance condition of the bridge the sum of the phase angles of the arms 1And 4 is equal the sum of the phase angle of arms 2 and 3.at the balance in condition thereis no current flow in the galvanometer.The balance equation is derived in the usual manner, and by substituting theCorresponding impendence and the admittance the value of the unknown capacitor andThe resister is finding as given below.Cx=C3 (R1/R2).Rx=R2 (C1/C2)Procedure: -1) Study the circuit provided on the front panel on the kit.2) Connect the unknown capacitance of the position given.3) Set the null point of galvanometer by adjusting the variable4) Calculate the value of unknown capacitance by formula givenResult: -The values of unknown capacitance is measured by shearing bridge

Construction and Working principle of LVDT Transducer: Construction: An LVDT is composed of seven components, not including the conditioning circuitry:

Primary coil

Secondary coil 1 Secondary coil 2 Ferromagnetic core Shaft Shield Handle

The ferromagnetic core is the moving component whose position within the shaft is sensed. Around the shaft are wound three inductors, the primary winding in the middle and the secondary windings (wound in opposite direction) on either side of the primary. Each of the secondary should have the same number of turns and be of the same length, otherwise the null position and linearity will be affected. A cylindrical shield protects the windings from damage and also serves to contain the magnetic field used for sensing. The physical component which will be measured using the LVDT is mechanically coupled to the ferromagnetic core using a threaded (and non-ferromagnetic) handle. For some applications, a guide system and even a spring-return (which is known as a “gage head” configuration) may be included in the assembly.

Working Principle of LVDT

LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is

converted into an electrical energy. And the way how the energy is getting converted is described in working

of LVDT in a detailed manner.

When an AC excitation signal is applied to the Primary Coil (P), voltages are induced in the two secondary

coils (S). The MAGNETIC CORE inside the COIL WINDING ASSEMBLY provides the magnetic flux path

linking the Primary and secondary Coils. Since the two voltages are of opposite polarity, the Secondary

Coils are connected series opposing in the center, or Null Position. The output voltages are equal and

opposite in polarity and, therefore, the output voltage is zero. The Null Position of an LVDT is extremely

stable and repeatable. When the MAGNETIC CORE is displaced from the Null Position, an electromagnetic

imbalance occurs. This imbalance generates a differential AC output voltage across the Secondary Coils

which is linearly proportional to the direction and magnitude of the displacement. As shown in the figure,

when the MAGNETIC CORE is moved from the Null Position, the induced voltage in the Secondary Coil,

toward which the Core is moved, increases while the induced voltage in the opposite Secondary Coil

decreases.

Electrical Strain Gauge:

Strain Gauge is a passive transducer that converts a mechanical elongation or displacement produced due

to a force into its corresponding change in resistance R, inductance L, or capacitance C. A strain gauge is

basically used to measure the strain in a work piece. If a metal piece is subjected to a tensile stress, the

metal length will increase and thus will increase the electrical resistance of the material and vice versa. This

will also change the electrical resistance of the conductor. If both these stresses are limited within its elastic

limit, the metal conductor can be used to measure the amount of force given to produce the stress, through

its change in resistance.

TypesStrain gauge transducers are broadly classified into two. They are

1. Electrical Resistance Type Strain GaugeIn an electrical resistance strain gauge, the device consists of a thin wire placed on a flexible paper tissue and is attached to a variety of materials to measure the strain of the material.  When a force is applied on the wire, there occurs a strain (consider tensile, within the elastic limit) that increases the length and decreases its area. Thus resistance of the wire changes, this change in resistance is proportional to the strain and is measured using a Wheatstone bridge. A full bridge circuit is used in applications where complimentary pair of strain gauges is to be bounded to the test specimen. In practice, a half bridge and full bridge circuit has more sensitivity than the quarter bridge circuit. But since, the bonding is difficult, a quarter bridge circuits are mostly used for strain gauge measurements. A full bridge circuit

is said to be more linear than other circuits.

An external supply is given to the bridge as shown in the diagram. Initially, when there is no application of strain, the output measurement will be zero. Thus, the bridge is said to be balanced. With the application of a stress to the device, the bridge will become unbalanced and produces an output voltage that is proportional to the input stress.

2. Variable Inductance Type Strain Gauge:

he basic arrangement of a variable inductance strain gauge is shown below. This type of strain gauge is very sensitive and can be used to measure small changes in length – as small as 1 millionth of an inch. Thus, it is highly applicable as a displacement transducer.

Instruments: Instruments are devices which are used for the measurements of quantities. Electrical

measuring instruments are devices which are used for the measurement of various electrical aspects such

as the presence and amount of current, voltage, resistance and power through installation. Some electrical

measuring instruments that measure one of the electrical aspects mentioned are ammeters, voltmeter and

ohmmeters.

Types of Instruments: Different types of electrical instruments are given below:

Analogue instruments

Moving-iron instrument

The moving-coil rectifier instrument

Electronic instruments

Electro dynamic (or Dynamometer) type Instruments

Secondary Instruments:1) Indicating instruments2) Recording instruments3) Integrating instruments

Permanent Magnet moving coil Instruments:

The permanent magnet moving coil instrument or PMMC type instrument uses two permanent

magnets in order to create stationary magnetic field. These types of instruments are only used for

measuring the dc quantities as if we apply ac current to these type of instruments the direction of current

will be reversed during negative half cycle and hence the direction of torque will also be reversed which

gives average value of torque zero. The pointer will not deflect due to high frequency from its mean position

showing zero reading. However it can measure the direct current very accurately.

Let us move towards the constructions of permanent magnet moving coil instruments. We will see the

construction of these types of instruments in

five parts and they are described below:

1) Stationary part or magnet system:

magnets are used of high field

intensities, high coercive force. The

magnets which we are using nowadays

are made up of materials like alcomax

and alnico which provide high field

strength.

2) Moving coil: The moving coil can freely

moves between the two permanent

magnets

3) Control system: The spring generally acts as control system for PMMC instruments.

4) Damping system: The damping force hence torque is provided by movement of aluminum former in the magnetic field created by the permanent magnets.

5) Meter: Meter of these instruments consists of light weight pointer to have free movement and scale

which is linear or uniform and varies with angle.

Permanent Magnet moving coil Instrument used as a voltmeter:

When PMMC is used as a voltmeter, the coil is connected in series with high resistance.

Permanent Magnet moving coil Instrument used as an ammeter:

When PMMC is used as an ammeter, except for a very small current range, the moving coil is connected

across a suitable low resistance shunt, so that only small part of the main current flows through the coil.

The shunt consists of a number of thin plates made up of alloy metal, which is usually magnetic and has a low

temperature coefficient of resistance, fixed between two massive blocks of copper. A resistor of same alloy is

also placed in series with the coil to reduce errors due to temperature variation.

Essentials of an electronic instrument

As shown Figure, an electronic instrument is made up of the following three elements:

Transducer: It is the first sensing element and is required only when measuring a non-electrical quantity, say, temperature or pressure. Its function is to convert the non-electrical physical quantity into an electrical signal. Of course, a transducer is not required if the quantity being measured is already in the electrical form.

Signal modifier: It is the second element and its function is to make the incoming signal suitable for application to the indicating device. For example, the signal may need amplification before it can be properly displayed. Other types of signal modifiers are: voltage dividers for reducing the amount of signal applied to the indicating device or wave shaping circuits such as filters, rectifiers or chopper etc.

Indicating device: For general purpose instruments like voltmeters, ammeters or ohm meters, the indicating device is usually a deflection type meter as shown in Figure. In digital readout instruments, the indicating device is of digital design.

Measuring Standards of an electronic Instrument:

These are the followings:

All the measuring electronic instruments are calibrated at the time of manufacture against

measurement standards.

Its measuring error is very less and can be neglected.

It uses low power

The size of electronic instrument is very small

The time it takes for the measuring is less and hence show the result quickly.

Electrical Measuring Transducers:

      In exact sense of word a measuring transducer is a device which will transform the change of one size in a change other. In terms of electronics a measuring transducer is determined usually as a device, transducers an unelectric physical size (called a physical measurand) in an electric signal, or vice versa. Present, certainly and exceptions from this rule      It is necessary from here, that measuring transducers are used in the electronic systems, i.e. in technical devices p. by an electric signal, representing the result of measuring or supervisions. On the other hand, a measuring transducer can be used on the output of the system that, say, to generate mechanical motion depending on an electric managing signal. The example of realization of transducers is the system in which a microphone (entrance transducer) converts a sound (physical measurand) into an electric signal. The last increases, and then acts on a loud speaker (output transducer), playback a sound substantially more loud, than that which is perceived by a microphone.

Maxwell’s Induction Bridge:

The Maxwell's Inductance Bridge is most commonly used bridge for measurement of inductance so f Q value below 10. A typical Maxwell’s bridge consists of an inductance measured in comparison with a capacitance in laboratory operations. The input for the bridge is given through a standard 1 KHz oscillator circuit which produces a 1 KHz sine wave at constant amplitude. The basic circuit diagram is as shown below.

In this bridge the arms bc and cd are purely resistive while the

phase balance depends on the arms ab and ad.

Here l1 = unknown inductor of r1. l2 = variable inductor of

resistance R2. r2 = variable electrical resistance. As we have

discussed in Ac Bridge according to balance condition, we have

at balance point

We can vary R3 and R4 from 10 ohms to 10,000 ohms with the help of resistance box.

Piezoelectric Transducer:Piezoelectric transducers are a type of electroacoustic transducer that converts the electrical charges produced by some forms of solid materials into energy. The word "piezoelectric" literally means electricity caused by pressure. Explanation:The piezoelectric transducers work on the principle of piezoelectric effect. When mechanical stress or forces are applied to some materials along certain planes, they produce electric voltage. This electric voltage can be measured easily by the voltage measuring instruments, which can be used to measure the stress or force.The physical quantities like stress and force cannot be measured directly. In such cases the material exhibiting piezoelectric transducers can be used. The stress or the force that has to be measured is applied along certain planes to these materials. The voltage output obtained from these materials due to piezoelectric effect is proportional to the applied stress or force. The output voltage can be calibrated against the applied stress or the force so that the measured value of the output voltage directly gives the value of the applied stress or force. In fact the scale can be marked directly in terms of stress or force to give the values directly.

The voltage output obtained from the materials due to piezoelectric effect is very small and it has high impedance. To measure the output some amplifiers, auxiliary circuit and the connecting cables are required.

Wheatstone bridge:We can measure unknown resistance by Wheatstone bridge using the following procedure.

1. Connect the unknown resistor, Rx, to the terminal of the

bridge.

2. Set the scale of the detector (galvanometer) to the least

sensitive range to prevent damage to the detector if the

bridge is severely unbalanced.

3. Connect the excitation source and adjust the resistor dial

until a null is obtained (zero deflection of the galvanometer).

4. If not at the most sensitive scale, increase the sensitivity of the galvanometer and repeat step 3;

otherwise, proceed to step 5.

5. Calculate the unknown resistance with equation R1Rx = R2R3. By substituting an AC source and an

AC detector, it is possible to measure the value of a resistor at the frequency at which it will be used.

Extension range of an Instrument:

The range of an instrument can be extended by the using the following methods:

In ammeter by connecting a shunt resister

In voltmeter by connecting a series resister.

Kilowatt hour meter:

kWh meter is the electric meter that measures the amount of electrical energy in kWh that was consumed in

the house. The kWh meter has a counter display that counts units of kilowatt-hour (kWh). The energy

consumption is calculated by calculating the difference of the counter's reading in the specified period.

Kilowatt-hour is an energy unit (symbol kWh or kW·h).One kilowatt-hour is defined as the energy consumed by power consumption of 1kW during 1 hour:

1 kWh = 1kW · 1h

The symbol of Kilowatt hour meter is .

Maximum demand indicator:

Maximum Demand Indicator (MDI) is an instrument which measures the maximum amount of electrical energy

required by a specific consumer during a given period of time.

Construction:

MDI instruments are designed in such a way that they record the base load requirement of electrical energy. They can also measure the peak load but are unable to record sudden short circuit or High motor Starting Currents. Its main construction parts are:

1. A Dial connected with moving system

2. A pointer on dial

3. Reset device

4. Fraction device

5. Indicating pin

Maximum demand indicator is often available as a built in feature of three phase energy meters, included in a single case.

Maximum Demand is calculated by

Maximum Demand (KW) = Maximum Energy Recorded (KWh) ⁄Time (hours)

Difference between primary and secondary instruments:

Primary Instruments Secondary InstrumentsThese instruments give the value of the electrical quantity in terms of absolute quantities (or some constants) of the instruments and their deflections.

They are direct reading instruments. The quantity to be measured by these instruments can be determined from the deflection of the instruments.

In this type of instruments no calibration or

comparison with other instruments is necessary.

They are often calibrated by comparing them with either some absolute instruments or with those which have already been calibrated.

They are generally not used in laboratories and are seldom used in practice by electricians and engineers. These instruments are used in general for all

laboratory purposes.Some of the examples of absolute instruments are:* Tangent galvanometer* Raleigh current balance* Absolute electrometer

Some of the very widely used secondary instruments are: ammeters, voltmeter, wattmeter, energy meter (watt-hour meter), ampere-hour meters etc.

Various methods of supporting the moving system in an instrument:

There are two methods of supporting the moving system:

a) By pivoting

b) By thread suspension

Pivoting:

If the ends of the spindle are conical, the bearing friction will

be more. To reduce the bearing friction the bearing is slightly

rounded off to reduce the contact area which will reduce the

bearing friction but the pressure on the ends of the spindle will

exceed 80 T/m2 when the ends are very sharp. To reduce the

pressure on the ends, the pivot is slightly sounded off.

Materials used are sapphire jewels (for bearings).

Thread suspension:

Thread suspension is advantageous when the operating

forces are small compared to the weight of the system. This

type completely eliminates bearing friction. Thread suspension also provides controlling torque in addition to

supporting.

Multimeter:

A multimeter is an instrument used to measure electric current, voltage, and usually resistance, typically

over several ranges of value. A multimeter, therefore, is a combination of several different types of meters

all in one box. At the minimum, a multimeter combines three distinct types of meters (ammeter, voltmeter,

and ohmmeter) into a single device. A multimeter may be

digital or analog.

Measurements with Multimeter:The face of a digital multimeter typically includes four components:

Display: Where measurement readouts can be viewed. Buttons: For selecting various functions; the options vary

by model. Dial (or rotary switch): For selecting primary

measurement values (volts, amps, ohms). Input jacks: Where test leads are inserted.

First of all set the dial (or rotary switch) for selecting the measurement for voltage, current or resistor.

Connect leads negative and positive from the input jacks with the given device same polarities.

Hence the required result will be shown on the display.

AC and DC bridges and its uses:

The D.C bridges are used to measure the resistance while the A.C bridges are used to measure the

impedances consisting capacitance and inductances. The D.C bridges use the D.C voltages as the

excitation voltage while the A.C bridges use the alternating voltage as the excitation voltage.The two types of D.C bridges

1. Wheatstone Bridge

2. Kelvin Bridge

The various types of A.C Bridges are:

1. Capacitance Comparison Bridge

2. Inductance Comparison Bridge

3. Maxwell’s Bridge

4. Hay’s Bridge

5. Anderson Bridge

6. Schering Bridge

7. Wien Bridge

Oscilloscope: An oscilloscope is a laboratory instrument commonly used to display and analyze the waveform of electronic signals. In effect, the device draws a graph of the instantaneous signal voltage as a function of time.

A typical oscilloscope can display alternating current (AC) or pulsating direct current (DC) waveforms having a frequency as low as approximately 1 hertz (Hz) or as high as several megahertz (MHz). High-end oscilloscopes can display signals having frequencies up to several hundred gigahertz (GHz). The display is broken up into so-called horizontal divisions (hor div) and vertical divisions (vert div). Time is displayed from left to right on the horizontal scale. Instantaneous voltage appears on the vertical scale, with positive values going upward and negative values going downward.

Function of oscilloscope:

The functions of oscilloscope are the followings:

1) Displays amplitude, frequency of one or more input signals.2) Compares amplitude, frequency, phase of two or more input signals. 3) Shows time relationship of start and stop of two or more input signals. 4) Shows time constant of input signals.

Potentiometer:

It is an instrument used for the measurement of electromotive force by balancing it against the potential difference produced by passing the known current through a known variable resistance.

A variable resistor with a third adjustable terminal, the potential at the third terminal can be adjusted to give any fraction of the potential across the ends of the resistor.

The measuring instrument called a potentiometer is essentially a voltage divider used for measuring electric potential (voltage); the component is an implementation of the same principle, hence its name.

Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick. Potentiometers are rarely used to directly control significant power (more than a watt), since the power dissipated in the potentiometer would be comparable to the power in the controlled load.

Meggars:It is an instrument which is used for the measurement of resistance of electrical insulation.The device enable us to measure electrical leakage in wire, results are very reliable as we shall be passing electric current through device while we are testing. The equipment basically use for verifying the electrical insulation level of any device such as motor, cable, generator winding, etc. This is a very poplar test being carried out since very long back. Not necessary it shows us exact area of electrical puncture but shows the amount of leakage current & level of moisture within electrical equipment/winding/system.