Electrical Heights

Detail of Electric Shock

2023-01-25T01:33:00.000+05:30

Electric Shock

Reasons for Getting Electric Shocks.

The human failure in handling the electrical components or repairing the distribution system without switching off the supply, the ultimate effect of electric shock on human body may be death.

Electric shock may result in:

Fibrillation of heart i.e. damaging the heart to small pieces causing the stopping of breathing.

Stopping of breathing action due to blockage in the nervous system causing respiration difficulties.

Local high heating or burning of the body due to sparking.

Death of a person.

First cause is dangerous and may put to death of person. Due to cause second and third there is a possibility of rescuing a person.

When an electric shock by human, the current passes through the body.

Strength of current passing through the body may result as shown below

Current and shock effect

1 mA. - Perceptible,No-pains.

1-4 mA - Tolerable, Not painful,Can be sensed.

4-15 mA - Medium but painful, but no-loss on muscular control.

15-20 mA - Painful, muscular control of adjacent muscles lost.

20-50 mA - Severe muscular contraction, breathing becomes difficult.

50 mA-100 mA - May result in ventricular fibrillation. May result in death.

100 to 200 mA - Heart stops, instant death

Above 200 mA - Severe burns, Sever chest muscles clamp-down, Heart stops,Sudden death.

Severity of Shock

"Do not touch to any Electrical application with a wet body".

Severity of Shock also depends on :

Frequency of current : Less frequency the shock is more severe.

Path taken by the current through the body:

If path is not through chest and heart then survival is possible

But if the path is through chest/heart then it is dangerous.

Duration of contact with electric installation:

More time duration contact is dangerous.

Person should be departed from the contact of machine immediately with a insulated thing.

Area of contact :

More contact area more is the risk.

Pressure :

More pressure of the body part with leaked m/c dangerous is the shock.

Physical condition of a person.

How much current in mA/sec is passed in the body.

Safety Signs and Symbols used in Industries

2021-02-23T12:25:00.000+05:30

Industrial Safety signs:

Warning sign - 
Warning sign

a sign giving warning of hazard or danger

Overhead load -  
Overhead load

There material handling going by Crain overhead

Caution - 
Caution

There is obstacle in the way

Flammable material

The material flammable.

Direction signs - 
Direction signs

Used for route direction

Raised platform or height restriction -

Emergency fire exit - 

Emergency fire exit

First-aid facility - 

First-aid facility

Fire extinguisher - 

Fire extinguisher

Different types of fire classes and their prevention:

Class A Fires - 

Class A Fires 

Fire involving combustible material like wood, paper, cloth, rubber, plastic requiring the heat absorbing effects of water, water solutions.

Class B Fires - 

Class B Fires

This type of fire involves flammable or combustible liquids  greases, petroleum products and' similar materials for extinction, a blanketing effect is essential.

Class C Fires - 

Class C Fires

Which involves the flammable gases, substance under pressure including liquefied gasses. So it is necessary to dilute the burning gasses at very fast rate with an inert gas, Dry chemical powder or carbon-di-oxide. 

Class D Fires - 

Class D Fires

Fire involving combustible metals like sodium, magnesium, zinc, potassium. These burning metal is react with water and water containing agent. fire requires special media to extinguish such as carbon-di oxide special  dry chemical powder.

Basic Detail of Power Factor

2021-02-23T10:16:00.003+05:30

Power Factor

What is Power Factor ?

It is the ratio of the active power of the fundamental wave the apparent power of the fundamental waveform

Cause:

Inductive load which cause large difference of phase angle between current and voltage,Varying load, industrial heating furnace, harmonic currents, electrical discharge lamp, AC motor, arc welder, furnaces, fluorescent lighting and air conditioning.

Effect:

overheating of motor and poor lightening.

Power factor correction:

This is normally achieved by the addition of capacitors to the electrical network which compensate the active power.
To reduce losses in the distribution system, and to reduce the clectricity bill, power factor correction, usually in the form of capacitors, is added to neutralize as ch of the magnetizing current as possible.
Capacitor included leads the voltage, producing a leading power factor.
The corrected power factor will be 0.92 to 0.95.

Why to improve Power factor?

If we use high power factor Reduction of I2 R losses in transformers and distribution equipment
Reduction of voltage drop in long cables
Extended equipment life
Extra kVA available from the existing supply
Increases the efficiency by reduction in power consumption
PF can be improved by (i) adding capacitor in parallel with the connect during the time of installation (ii) adding static power factor correction

Benefits of Power factor correction:

Improved energy efficiency
reduced system currents and kW losses
Increase system load without the invest in additional infrastructur
Environmentally friendly - reduced kWh losses mean that less power ne generated, so less CO2 is produced
Increased equipment life

Basic Principles of Transducer

2019-11-01T14:20:00.000+05:30

Basic Principles of Transducer

Pressure Transducer

The task of a process control system is to control the physical parameters like force, pressure, level etc and to maintain their value near the desired value.This is because the physical parameters may vary due to disturbances.

To maintain these parameters at a required level certain corrective action requires to be taken.

Fig. shows a process control loop.

Process control loop

The process control loop consists of four blocks process,parameter measurement,
control element and controller.

It is essential to measure the actual value of parameter P to achieve proper feedback.

Nowadays most of the controllers are electronic and hence they require an electrical input. Thus,the feedback signal is required in electrical form.

The measurement block includes two functions like to measure the parameter and to produce a suitable analog signal in electric form.

Sometimes the parameter measurement blocks produces non electric signals.Hence,we require a device that will convert this non electric signals to an electric signal.

Define transducer:

Transducer is converts a physical quantity to be measured into an equivalent electrical signal(voltage or current).

The physical quantity to be measured is like temperature,pressure,displacement,
vibration, flow, etc.

The electrical signal obtained from the transducer is then used to control the physical quantity automatically and/or to display the same,as shown in Fig.

Transducer

A transducer is also called as a pick up element.

It contains two parts that are closely
related to each other i.e the sensing or detecting element and the transduction element.

The sensing or detecting element is called as the sensor.It is a device producing measurable response to change in physical conditions.

The transduction element who converts the sensor output to suitable electrical form.

Fig. shows block diagram of transducer.

Transducer block diagram

Generally the range of electrical signal used is 0 to 5V or 4 to 20 mA,4 to 20 mA range is commonly used to represent an analog signal.

A current of 4 mA represents zero output while a current of 20 mA represents a full
scale value.The zero condition represents open circuit is the transmission line. Hence,the standard range is offset from zero.

Sometimes the transducer is a part of system and works with other elements to achieve desired output.This kind of circuit is called as a signal conditioning circuit.

Classification of Instrumentation System

2019-10-30T13:10:00.000+05:30

Classification of Instrumentation System

Instrumentation system

Null and Deflection type Instruments:

Deflection type instruments:

The instruments in which deflection of the instrument forms the basis of measuremenl are called as deflection type instruments.

The example of deflection type instruments is the permanent magnet moving coil (PMMC) ammeter.

PMMC AMMETER

The deflection of the coil is proportional to the current that is to be measured.

Torque Td, that acts on the moving coil is proportional to current.The opposing effect is produced by a spring, having torque Tc which is proportional to the deflection.

When the spring is balanced Tc=Td.

The value of the current depends on the deflection.Thus,the value of the measured quantity depends upon the calibration of the instrument.

Null type Instruments

The instrument in which, a null or zero indication will lead to the determination of the magnitude of the measured quantity such instruments are called null type instruments.

The null condition depends on other known conditions.

The DC potentiometer is an example of null type potentiometer.

Fig. shows the D.C.potentiometer.

DC potentiometer

The unknown emf is to be measured.The slide wire of the potentiometer is calibrated in terms of emf with the help of a standard emf source.

The null detector is a galvanometer whose deflection is proportional to the unbalance emf.When the difference between the two sliding contacts is zero, no current flows through the galvanometer and it will show zero deflection.

Thus,the unknown emf is calibrated directly on scale.

Analog and Digital Instruments:

Analog Instruments:

The signals which continuously vary and takes infinite values for a given range are called as analog signals.The instrument which produces analog signals is called as an analog instrument.

In analog system the function continuously varies.

Examples of analog instruments are deflection type instruments like PMMC ammeter, voltmeter, null deflection instruments like DC potentiometer, wheatstones meter bridge etc.

Digital Instruments:

The signals which vary in discrete steps and take a finite different values in the given range are called digital signals.

The instrument which produces digital signals is called as digital instrument.

Examples of digital instruments are logic analysers,microprocessor based instruments, computers etc.

Active and passive Instruments

2019-10-29T10:15:00.000+05:30

Active and Passive Instruments:

Active and passive Instruments

Passive Instruments:

The instruments where the output is produced completely by the quantity that is measured are called passive instruments.

The example of passive instrument is a pressure gauge.Fig shows a pressure
gauge.

Passive Instrument

As the pressure in the liquid changes, the piston moves.The pointer is connected to the piston.

No other source of energy is used. The liquid pressure is the only source.

Thus,the piston and pointer movement while measurement is entirely dependent on the liquid pressure.

Active Instruments:

Active instruments are the instruments in which the quantity to be measured activates the magnitude of external power input source that produces the measurement.

The liquid level indicator is an example of an active instrument.Fig.1.4.2 shows a
liquid level indicator.

Active Instruments

The power source is required in order to sense the position of float.

The position of float is proportional to the liquid level of the tank.As the level of tank
changes,the float position also changes.

The variable to be measured changes the value of the power source, as the change in float level changes the slider potentiometer also moves.This in turn changes the external power input.

Advantage of passive Instruments

They are simple to design.

They are cheap.

Disadvantage of passive Instruments

The resolution is less and resolution cannot be easily increased

Some passive Instruments are pressure gauge, glass thermometer, voltmeter

Advantage of Active Instruments

The resolution is adequate and can be
adjusted by adjusting magnitude of
external energy input.

Disadvantage of Active Instruments

They are complex in design.

They are costly.

Some Active Instrument is liquid level indicator

Instrumentation System

2019-10-28T14:22:00.001+05:30

Functional elements of an Instrumentation System:

Instrumentation system

Any instrumentation system can be described with the help of a block diagram.The block diagram of an instrumentation system describes the essential elements and their
functions in a measuring system.

Fig. shows the block diagram showing the functional elements of an instrumentation system.

Instrument system

The elements can be grouped as:

Primary sensing element.
Data conditioning element.
Data presentation element.

Each element comprises of a number of distinct components that perform a particular function in the measurement procedure.The function of each element is important.

Primary Sensing Element:

To the primary sensing element the quantity to be measured is applied.e.g.in an ammeter, coil carrying current to be measured is a primary sensing element.

Generally a transducer follows a primary sensing element.The transducer converts the measured to corresponding electrical signal.

Variable Conversion Element:

The output from the primary sensing element is in electrical form like voltage,
frequency or any other electrical parameter.

Such output is not suitable for the actual measurement system.e.g. if the measurement system is digital then the analog electrical signal obtained from the primary sensing element is not suitable for the digital system.

Hence a variable conversion element like A/D converter is required.

Some instruments may require more than one variable conversion element.While conversion the original data must be retained.

Variable Manipulation Element:

The level of output from the earlier stage may not be sufficient enough to drive the next stage. Thus, the variable manipulation element manipulates the signal, preserving the original nature of the signal.

The variable manipulation element can be placed before the variable conversion element if required.

If the level of signal obtained from earlier stage is large then it must be reduced.In such cases attenuators are used as variable manipulation elements.

In some cases the output of transducer is affected due to unwanted signals like noise. These signal must be processed by some processes like clipping, clamping etc. To achieve the signal in undistorted from This process is called as signal conditioning.

Data Transmission Element:

When the elements of the system are physically separated , it is essential to transmit the data from one stage to other. it is obtained by a data Transmission Element

Data Presentation Element:

Data that is transmitted is used by the system for monitoring, controlling or analysing purpose. thus the person who is handling the instrument must get the information in proper form,according to the purpose for which it is required. This function is done by a data Presentation element.

Definition of Measurement and Calibration

2019-10-27T12:44:00.001+05:30

Definition of Measurement and Calibration

Measurement and Calibration

Measurement:

Measurement of a given quantity is the act or result of a quantitative comparison between an unknown quantity and a predefined standard quantity
e.g.for measuring the weight of a quantity, the standard taken is 1 kg.

Measurement using an instrument as a physical means of determining a quantity or variable.

Instrument:

Define an instrument:
Instrument can be defined as a device for measuring the value or magnitude of a quantity or variable.

The electronic instrument is based on electrical or electronic principles for their
measurement.

Calibration:

What do you mean by calibration?

Calibration is a process of estimating
the value of a quantity by comparing that quantity with given standard quantity.

The standard with which comparison is made is called as standard instrument.

The unknown quantity is to be calibrated.This quantity is called as test
quantity. If an instrument is to be calibrated,it is called as test instrument.

For calibration,the test instrument will be compared with the standard instrument.

The calibration offers a guarantee to the device or the instrument that it is operating Witn required accuracy, under the stipulated environmental conditions.

Calibration creates the confidence of using properly calibrated instrument in the user's mind.

It is essential to calibrate an instrument from time to time.

The calibration process deals with visual inspection of defects, zero adjustment etc.

By applying known values of quantities to be measured and recording the corresponding output,the calibration characteristics of an instrument can be determined.The output values are compared with the input values to estimate the error in measurement.Such a record obtained from calibration is called as calibration record.

The calibration record is recorded in tabular form or in graphical form.If it is recorded in graphical form it is called as a calibration curve.Such a calibration record is useful to obtain the characteristics of an instrument.

Steps for Calibration:

The following step is necessary for Calibration

1. Examine the instrument construction and identify all the possible input.

2.Make a decision about the appropriate input required for Calibrating the instrument.

3.Procure apparatus that will allow user to vary the input over the rang. Procure the standards to measure each input.

4.Make some of the input constant and vary the other and record the output and develop the desired static input-output relations.

CLASSIFICATION OF NETWORKS

2019-10-26T12:34:00.000+05:30

CLASSIFICATION OF NETWORKS

Classification of Network

The electrical networks may be classified in a number of ways depending upon following.

(i)the kind of elements of which the network is composed of and on the basis of their properties

(ii)in terms of the general properties of its responses to excitation applied to its input terminals.Thus,we have linear and non linear networks, time invariant and time variant networks, active and passive networks etc.

Linearity:

In a linear network, the voltage-current relationship is show by a linear
equation.

Suppose the network to be classified is under relaxed condition such that the initial condition is zero and an excitation e1(t)is applied for which the response is c1(t)and for excitation e2(t), the response is c2(t).Then the network is classified as linear if for excitation e1(t)+e2(t), the response is c1(t)+c2(t).This shows that a linearity follows superposition principle.

Hence,from physical point of view, it may be said that a necessary condition
for a network to be linear is that the principle of superposition applies.

The validity of the principle of superposition means that the presence of one excitation does not affect the responses due to other excitations and there are no interactions amongst the responses of different excitations in the network.

Hence,we conclude that a network is linear if it satisfies the above principle
of superposition;otherwise it is nonlinear.

Time-Invariance:

A network is said to be time-invariant if the network produces the same
Response to a given excitation irrespective of time of application of excitation.Thus, If the response to an excitation e(t) is c(t). then in time-invariant network,the excitation e(t+t,) will produce a response c(t+t1) where t1 is any time. This implies that the values of network component is constant at all time and do not change with time in a time - invariant network

If the parameters vary with time than network is called a time - varying network

Reciprocity:

Some networks have the property that the response produced at one point of the
network by an excitation acting at another point is invariant if the positions of excitation and response are interchanged depicts a network having no initial energy storage.

GENERAL NETWORK

The network is excited by a voltage v1(t)at terminals 1-1'and the response is the current i2(t) in the short-circuited terminals 2-2'.In Fig the excitation is applied to the same network at terminals 2-2' and the response is the current in the short-circuited terminals 1-1' with the
same polarities.The network is said to be reciprocal,if and only if i1(t)=i2(t)when v2(t)=v,(t) otherwise,it is non-reciprocal.

Lumped and distributed networks:

The preceding classification of the networks was based on the basic behaviour of the response of the network. However networks may be classified in a number of other ways such as lumped and distributed networks,one-port and two-port networks,unilateral and
bilateral networks etc.

Unilateral and bilateral network:

In bilateral element the voltage current relation is the same irrespective of direction of flow of current in it e.g.resistance,inductance,capacitance etc.In constract, a unilateral
element has different voltage-current relations for two possible directions of flow of current such that diode,rectifier etc.

Types of Electrical Energy

2019-10-25T12:19:00.001+05:30

TYPES OF ELECTRIC ENERGY:

Electrical Energy

in the methods of science,we immediately become suspicious of any scheme who claims to create energy.

The law of conservation of energy is that energy can neither be created nor destroyed.

However, its form can be converted such that energy can be converted from one form of energy to another.

Electric energy is the energy who converted from some other form.
There are a numbers of way in which this is accomplished.

Some of them are as follows:

(1)Electrochemical energy conversion:

Electric batteries produce electric energy with the help of conversion of chemical energy.

A potentially important use of this type of batteries is in the electric car. Fuel cell is in this general classification.

(2)Electromechanical energy conversion:

The rotating generator produces electric energy from mechanical energy of
rotation.

Usually, the mechanical energy who converted from thermal energy by a turbine and, in turn, the thermal energy is converted from chemical energy with the help of burning fossil fuel or from nuclear fuel.Sometimes.the conversion is from hydrauilic energy by hydroelectric generation.

(3)Magnetohydro dynamic (MHD) energy conversion:

These devices make electric energy from the mechanical energy of a high velocity ionised gas.

(4)Photovoltaic energy conversion:

A class of devices are able to convert light energy directly in to electric energy.
The best known device of this types are the solar cell.

(5)Thermo-electric energy conversion:

When two dissimilar metals say copper and iron are joined together at two ends
and a temperature difference is maintained between the joints an electric current flows between the junctions.

All these devices may be called energy sources. The function of each of these
different sources of electric energy is the same in terms of energy and charge.in one form of battery, for example, two metallic electrodes one of zinc and one of copper are immersed in dilute sulphuric acid.

The formation of zinc and copper ions causes negative charge to accumulate at the electrodes.Energy is supplied to the charge by the difference in the energy of ionization of zinc and copper in the chemical reaction.

Once the battery circuit is closed by an external connection the chemical energy is expended as the work for each unit of charge in transporting the charge around the external circuit.

The quantity " energy per unit charge ” or identically, " work per unit charge ” is called voltage.

BASIC OF CHARGE AND CURRENT

2019-10-24T14:05:00.000+05:30

CHARGE AND CURRENT:

CURRENT

concept of charge is based on atomic theory.

We know that all matter is made up of fundamental building blocks called atoms and that the atoms,in turn,are composed of different kinds of fundamental particles.

The three most important particles are the electron,the proton,and the neutron.

The electron possesses negative charge,the proton possesses equal-magnitude positive charge and the neutron have neutral or has no charge at all.

As we rub the rubber comb with woolen
cloth,the comb acquires its negative charge because some of the electrons on the wool are rubbed off on to the comb;the cloth then has an insufficient number of electrons to maintain its electrical neutrality and thus behaves as a positive charge.

When electrons are removed from the substance,that substance becomes a positively charged and the substance acquiring electrons from the other substance becomes a negatively charged.

The basic unit of the charge,called the Coulomb after Charles Coulomb,the first person to make careful quantitative measurements of the force between two charges.The electron has a charge of 1.6021 x10-19 Coulomb.

The phenomenon of transferring charge from one point to another pointis described by the term,electric current.An electric current may be defined as the time rate of net motion of electric charge across a cross-section of the conducting malerial.

Remember that random motion of free electrons in a metal does not constitute a current unless there is a net motion of the charge with time.

In equation form,the current is

i=dq/dt

If the charge q is in coulombs and time t is measured in seconds,then the current is measured in amperes.

Since the electron has a charge of 1.6021 x 10-19Coulomb, it follows that a current
of l ampere corresponds to the motion of 1/1.6021 x 10-19=6.24 x 1018 electrons
past any cross-section of a conducting path in one second.

A charge in motion constitutes an electric current.

Difference Between Slip ring and Squirrel cage induction motor

2019-10-20T15:23:00.002+05:30

Difference Between Slip ring and Squirrel cage induction motor

Difference Between Squirrel cage and Slip ring inductio motor

Slip ring or phase wound Induction motor

1.Construction is complicated due to presence of slip ring and brushes.

2.The rotor winding is similar to the stator
winding.

3.We can easily add rotor resistance by using slip ring and brushes.

4.Due to presence of external resistance high starting torque can be obtained.

5.Slip ring and brushes are present.

6.Frequent maintenance is required due to presence of brushes.

7.The construction is complicated and the presence of brushes and slip ring makes the motor more costly.

8.This motor is rarely used,only 10%
industry uses slip ring induction motor.

9.Rotor copper loss is high and so less efficiency.

10.Speed control by rotor resistance method is possible.

11.Slip ring induction motor.is used where high starting torque is required i.e. in hoists, cranes, elevator etc.

Squirrel cage induction motor

1.Construction is very simple.

2.The rotor consists of rotor bars which are permanently shorted with the help of end rings.

3.Since the rotor bars are permanently|
shorted,its not possible to add external
resistance.

4.Starting torque is low and cannot be|
improved.

5.Slip ring and brushes are absent.

6.Less maintenance is required.

7.The construction is simple and robust and it is cheap as compared to slip ring
induction motor.

8.Due to its simple construction and low cost the squirrel cage induction motor is widely used.

9.Less rotor copper losses and hence high efficiency.

10.Speed control by rotor resistance method is not possible.

11.Squirrel cage induction motor is used in lathes,drilling machine,fan,blower printing machines etc.

Voltage Regulator and Review of Basic DC-DC Voltage Regulators

2019-09-13T16:14:00.001+05:30

Voltage Regulator and Review of Basic DC-DC Voltage Regulators

The regulated output DC voltage can be obtained using either linear power supply or switched mode power supply.

Linear power supply consist of devices that operate in active regions to regulate the DC output voltage.

the block diagram of linear power supply is shown in below figure:

The transistor T1 is connected in series with the DC source voltage. Here note that the series transistor T1 operates in active region.

The error amplifier compare the output voltage Vo(av) with set value Vref.

The difference between Vo(av) and Vref is called error voltage. it is given to control circuit.

The control circuit generates the base drive of T1 in such a ways that error voltage is zero this means Vo(av)=Vref

The output voltage Vo(av)=Vs-Vce. This means the transistor T1 has the power loss of Vce x io. This loss takes place continuously across T1. Because of this reason, linear power supply has reduced power efficiency.

The efficiency of linear power supply is as low as 305 to 35%. The power loss is dissipated as heat in series transistor.

Advantages of Voltage Regulator

Low output ripple and noise
Fast Transient response
No RFI or EMI
Economical circuit for low power output

Disadvantages of Voltage Regulator

Reduce power efficiency
Bulky size
Bulky low frequency transformer is required
Large heat sink is required for power dissipation

Application of Voltage Regulator

DC power supplies
Linear Circuits

Basic of DC - DC Voltage Regulator

the DC - DC voltage regulator are classified as follow:

1. Linear voltage regulator
2. Switching voltage regulator
3. Resonant converter

The Switching voltage regulator are classified as below

1. Non isolated converter

There is no electrical isolation between supply and output

2. Isolated converters

The supply and output are electrically isolated via high frequency transformer.

Ohm's Law | Basic Law of Electrical | Terms Related to Electrical

2019-09-04T11:51:00.001+05:30

Ohm's Law | Basic Law of Electrical | Terms Related to Electrical

Ohm's Law

Ohm's law states that the current through any conductor is directly proportional

to the potential difference between its ends, if the physical conditions of the

conductor do not change.

Ohm's Law | Basic Law of Electrical | Terms Related to Electrical

That is, if temperature, length and cross sectional area of conductor are not

changed then,

I ∝ V

I = V / R

V=IR

where,

V=voltage between ends of the conductor in volts

I=current through the conductor in amperes.

The V - I characteristics of it is shown in below figure.

V - I characteristics

If a curve is plotted between V and I it is found to be a straight line.

All materials are broadly classified into three categories depending upon the

resistance they offer to the flow of current through them. The electrical properties of the materials largely depend upon the number of free electrons available.

Terms Related to Electrical

Conductor

It is a material which has a large number of free electrons so, the current can easily flow through it.

All materials with resistivity less than 10^-3 ohm meter behave as conductors such that silver, copper, aluminium, carbon and almost all metals.

Insulator

If the outer electrons of a material are very tightly bound to the nucleus, it

becomes very difficult to remove them from their orbits.

Hence current cannot flow through such materials and they are known as insulators.

All materials with resistivity above 10^5 ohm meter behave as insulators such that mica, porcelain, glass, rubber, oil, plastics, etc.

Semi-conductors

Materials like germanium and silicon are neither good conductors nor insulators.

They are called semiconductors.

Normally their outer electrons are bound

to the nucleus but the electrons can be made free by some means.

Example:

adding some impurity to germanium or silicon can make the outer electrons

free.

By increasing the temperature also these electrons can be made free.

Such materials have resistivities between 10^-3 and 10^5 ohm meter.