Download Tesla Coil P6-3305

Tesla Coil
P6-3305
HISTORICAL BACKGROUND
Nikola Tesla was a Yugoslavian who achieved most of his success as an inventor in the field of electricity and
magnetism in USA. Among his many inventions are included, a system of arc lightening (1886), the alternating
current motor, power generation and transmission systems (1888), system of electrical conversion and
distribution by oscillatory discharges (1889) and a generator of high frequency currents (1890). Tesla was
among the first persons to recognize the importance of alternating current and almost all of his achievements are
related to alternating currents only. In the year 1891, Tesla invented what is his most famous invention - the
tesla coil transformer for producing high frequency, high voltage alternating current and thus gave the basis for
the wireless transmission of electrical power.
After his invention of Tesla Coil, he continued his work on its improvement and applications. In the year 1899,
while working at Collrade Springs Laboratory in America, he produced a 135 feet long discharge, 200 feet
above the earth with a 12 million volt coil. This lead to the overloading of the power line and as a result set fire
to the alternator of the Colorado Springs Electric Co. His main purpose behind this invention and further
research on it was mainly for the wireless transmission of electric power.
In recognition of the Nikola Tesla’s contribution to the pure and applied science in the field of electricity and
magnetism, the unit of a magnetic quantity i.e., magnetic flux has been named after him as “Tesla”.
INTRODUCTION:
Tesla coil is a type of resonant transformer, which works on the principle of resonance of two inductor
capacitors (LC), circuits (i.e., primary and secondary LC coils). It produces high frequency voltages of very
high magnitude from high current, low voltage input and can be safely used to experiment/demonstrate the
physical phenomena associated with such type of outputs.
In a basic tesla coil, the two primary and secondary coils are tuned to the same frequency (i.e., natural frequency
of both the coils) so as to produce resonance. The circuit containing inductance and capacitance and very low
resistance (LC circuit) is oscillatory in nature and the frequency of its oscillations are very large. The natural
frequency of oscillations of LC circuit is given by
n=
1
2π LC
Where, L & C are inductance and capacitance of the circuit.
The primary and secondary coils have frequency of the order of 100KHz to 1MHz and are designed in such a
way so as to have highest possible Q-factor (lowest circuit loss or lowest bandwidth).
If one inductance is made primary of an air core step-up transformer, the voltage induced at the other inductance
(or secondary) is very high and since the frequency of alternation of both primary and secondary is same, it will
also be very high. Such an arrangement is the basis of Tesla coil. The attached diagram shows the schematic
representation of the Tesla coil.
Here G is a spark gap of about 2 to 3 mm and P is the inductance containing only a few turns of thick copper
wire which serves as the primary of a step up transformer. The secondary coil of this transformer ‘S’ contains
comparatively larger number of turns of thinner copper wire and is a narrower coil inside the primary coil P. No
iron is used and enough space is provided between the two coils in order to prevent sparking. One end of both
the primary and secondary coils is earthed and the other end of the secondary has such a high voltage that it
produces several inches long spark. Sparks produced from the coil can easily be taken on the naked hand
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without any harm. Due to the magnitude of potential being high, the frequency of alteration is also very high
and the high frequency alternating currents tend to travel over the surface of the conductor only. Hence, the
current due to the spark does not pass through the body, thus rendering it harmless. This is the ‘skin effect’.
In this case, the secondary capacitance Cs of the coil is predominantly a distributed capacitance (distributed over
the surface of the coil). This capacitance is due to the larger number of turns and the proximity of each turn in
the secondary winding. Thus the secondary behaves much like an open circuit transmission line. For designing
the tesla coil, the resonant frequency of the secondary is calculated in such a way that the frequency of
operation of tesla coil is the lowest of the resonant frequencies, in order to avoid voltage other than at the top of
the secondary coil.
For resonance, the frequency of oscillation of both the coils (n) should match.
⇒ n=
1
2π L p C p
⇒ L pC p = LsCs
∴
Lp
Ls
=
Cs
Cp
=
1
2π L s C s
............... (1)
............... (2)
............... (3)
If the coupling between both the primary and secondary coils is adjusted to be near the critical value (kQ∼1,
where k is coupling constant) then,
Vp
Vs
=
Lp
Ls
............... (4)
Where Vp & Vs are primary and secondary voltages respectively.
⇒ Vs = Vp
Ls
Lp
............... (5)
Also from (3),
Vs = Vp
Cp
Cs
............... (6)
In this case value of Vs does not depend upon the ratio of turns of the primary and secondary coil. Usually, the
secondary coil is made first and then the value of Cp, Lp and k is adjusted in such a way, so as to give the
maximum value of Vs for a given Vp.
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Tesla Coil:
The tesla coil apparatus is show in the attached diagrams. Diagram 1 shows the actual apparatus, whereas
diagram 2 is the schematic representation of the same.
The basic components of the apparatus are a vibrator (for supplying high frequency input), two high voltage
capacitors (of 0.047µF with current rating of 2000 VDC) connected in series with each other, and primary coil
and secondary coil connected to the threaded terminal at the top.
The vibrator consists of two major components. One is the hollow iron core on which copper wire has been
wound. This acts as an electromagnet on the passage of current. The other one is a buzzer with a spark gap. The
two tungsten contact points present in the buzzer are held together with the help of springs. When alternating
current is passed through them, the contact points open and close with the help of springs and accordingly allow
current to pass through the core resulting in the formation of a spark at the spark gap.
The primary coil of the apparatus consists of a few turns of thick insulated copper wire wound near the base, at
a gap, from the core of the secondary coil in such a way that there is no physical contact between the primary
and the secondary coil. Both the capacitors and the spark gap are connected to the primary coil in series.
The secondary coil of the apparatus is made up of enameled copper wire comparatively much thinner than the
primary with very large number of turns on a cylindrical insulated core (in the form of a solenoid). The large
number of turns of the secondary coil thus provides higher turn ratio or larger impedance, hence stepping up the
voltage to very high levels. The output of the tesla coil is obtained at the threaded terminal at the top of the coil
over the corona cap.
Functioning of Tesla Coil:
This tesla coil apparatus is designed to work at 110V/220V AC available from any power source. When the
electric current is passed, the hollow iron core of the vibrator starts behaving like an electromagnet, since
current carrying copper wire is wound around the core. The magnetic field produced by the iron core attracts
one of the tungsten contact points (which is nearer to it), thus separating the two. On the disintegration of
magnetic field, the tungsten contact point returns to its original position due to the action of the spring attached
to it. This is repeated with every reversal of the direction or polarity of the AC supply since it influences the
formation of electromagnet accordingly.
When one tungsten contact point is physically apart from the other contact point due to the magnetic action of
the vibrator, the current passes through the capacitors in order to complete the circuit and as a result the
capacitors get charged. But when the two contact points are in contact with each other (in the absence of
magnetic field), it shorts the two capacitors and does not allow the electric current to pass through them. The
gap formed between the tungsten contacts, in the earlier case, acts as a spark gap. The air present in the spark
gap gets ionized resulting in the formation of spark across the contacts. This spark, since it is carrying current,
short-circuits the transformer and the capacitors. This enables the flow of current through the gap; and at the
same time the capacitors don’t get discharged. Here, the spark is analogous to that of a high power high-speed
switch. The wider the spark gap, the higher the break down voltage is. In fact, no other device, regardless of
whether it is electronic or semiconductor, is capable of switching such high power levels as quickly as spark
gap. Once the voltage across the spark gap is sufficiently high, it ionizes the air present in the gap. As soon as
there are adequate number of ions present in the gap, it results in the spark gap becoming conductive and
electric charge in the form of spark travels across the gap, thus substantially decreasing the electrical resistance
across the gap. As the voltage across the spark gap decreases, the ionization level of the air also decreases, thus
increasing the electrical resistance considerably and the flow of electrical current stops. The spark gap now
starts behaving as an open circuit. Extinguishing of the spark is referred to as quenching. The spark gap that
© 2006 Arbor Scientific * P.O. Box 2750, Ann Arbor, MI 48106-2750 * (800) 367-6695 * ww.arborsci.com
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quenches well, stops conducting very quickly. Actually, the arc formed due to the spark in the spark gap is not a
single arc along one direction only. It is a series of a large number of different arcs, which is changing its
direction very frequently. But since the frequency of this oscillation is so high, it appears to be a single arc if it
is observed unaided. With each oscillation the direction of flow of current through the gap also changes. This
frequency is usually of the order of several kilohertz.
When the magnetic field produced by the iron core of the vibrator decays, the capacitors start discharging. The
charge stored in the capacitors is transferred to the primary coil, which in turn produces a magnetic field.
Presence of both electrical energy and magnetic energy results in the formation of electromagnetic field. In the
reverse cycle of the electric current, reformation of magnetic field of the iron core of vibrator takes place. At
this stage, the electromagnetic field generated by the primary coil starts decaying and capacitors get recharged
with the electrical charge of increasingly higher voltage. This is because every formation of the electric field
results in the accumulation of additional charges that have been generated previously. The vibrator, at the same
time also functions as an air core transformer, which increases the voltage to relatively medium level with every
change in direction of the current. It also does not allow the normal 60Hz supply to pass, owing to its design
considerations. The two capacitors connected to the primary coil are supposed to perform many functions. Apart
from the storage of electrical charge, they filter the high frequency electric current, i.e., they don’t allow the low
frequency electric current to pass through them. This also adds to the safety of the coil since the low frequency
high voltage currents are capable of giving electric shock when the coils are touched. This is due to the absence
of skin effect at low frequencies (in this case the current passes through the body also, thus producing electric
shock). This results in the generation of the very high frequency electrical energy in the primary coil.
As a result, when the frequency reaches a sufficiently high level and the voltage in the primary Vp equals the
one derived from the design considerations (as discussed above), the magnetic field due to primary coil
produces magnetic field in the secondary coil by the process of induction. When this frequency of oscillation in
both primary and secondary is equal to their natural frequency of oscillation (which is the same in this case),
resonance is produced. This resonance leads to the addition of both the amplitudes of the electric potentials,
resulting in the production of very high electric potential in the resonant circuit. At this stage the AC resistance
of the circuit (i.e., the reactive impedance of the LC circuit) becomes negligible.
Since the secondary coil consists of very large number of turns as compared to the primary coil, it behaves like
a transformer and enhances the primary voltage considerably. This depends more on the number of turns of
secondary coil within the range of electromagnetic field created by primary coil. Also, in the secondary coil,
very small amount of potential difference is there between each subsequent turn of the coil and it helps in
preventing high voltage from breaking down the insulation of copper wire.
In this case the frequency of oscillation is given as:
n=
1
2π L p C p
Lp = 1µH = 1x10–6 H
Here,
Cp =
⇒
1
1
1
+
−6
0.047x10
0.047x10 −6
Cp = 0.0235 x 10-6 F
∴n=
1
-6
2x3.14 1x10 x 0.0235x10 -6
= 1.0382 MHz
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Also, secondary voltage,
Vs = Vp
Cp
Cs
Vp ≈ 3000V;
Cp = 0.0235 x 10-6F
Cs ≈ 90 x 10-6 F
Vs = 3000
0.0235x10 −6
≈ 48,500V
90x10 −12
Along with the apparatus, an aluminum ball has been provided, that can be threaded into the terminal on top of
Corona. This aluminum ball has a highly polished surface and acts as the discharge terminal. It provides two
main functions.
1.
The aluminum ball (sphere) functions as a capacitance, which resonates with the secondary winding of
tesla coil. When the tesla coil is being used, the ball terminal ionizes the air surrounding it, resulting in
the further increase of capacitance. Other objects present in the vicinity of the terminal also enhance
the capacitance of the coil.
2.
The spherical shape of the terminal helps in the production of uniform electric field at the top all along
its surface, which results in the minimization of potential gradient around the top of the coil. Hence,
very high voltages on the terminal can be produced without worrying about the electrical stress being
created in the surrounding air. Thus higher voltages can be produced before the formation of the spark
and also the spark thus formed will be for more time duration.
Precautions while handling Tesla Coil:
1.
Although the output produced is fairly safe, even so, the apparatus should be used under expert supervision.
2.
Any person with a weak heart or with cardiac problems should not be allowed to come in contact with the
high voltage output.
3.
Even though high frequency high voltage output does not produce electric shock but if the area of contact
with your body is small, the high current density (due to concentration on a small area) may result in burns.
Discharging the coil with a piece of metal held close to the terminal may reduce this.
4.
The tesla coil should not be placed on a metal or conducting base while in use.
5.
Make sure that the earthing of the power supply is in proper shape and is connected to the earth of the tesla
coil.
6.
The plastic covers covering the base should not be removed, and also ensure, there is no loose worn or
frayed wire.
7.
The apparatus is not to be operated in a humid environment and should be used indoors only.
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Operating the Tesla Coil:
Following procedure can be adopted for operating the tesla coil apparatus and also to properly adjust the coil for
maximum output.
1. Take the tesla coil apparatus and thread the aluminum ball terminal on top of it.
2.
Take out the discharge electrode, which is an aluminum rod, bent at one end in the form of hook and fix it
on top of the bakelite terminal provided on the base of the coil.
3.
Plug the coil into 110V/220V AC power source using the connecting lead provided and switch on the
power supply so that the current starts flowing through the coil.
4.
Discharge will be observed between the aluminum ball terminal and the discharge electrode. Carefully
adjust the buzzer knob till the strength of the discharge is maximum and for longer duration. It implies the
tesla coil is producing maximum output.
BEHAVIOR OF DIFFERENT SUBSTANCES AT HIGH FREQUENCY AND HIGH VOLTAGE
Additional items required:
(a) Thin conducting wire
(b) Plastic piece
(c) Neon bulb with socket; or a metal part.
Procedure:
1.
Set up the coil as explained above with aluminum ball terminal and start operating it. No discharge will be
observed from the aluminum ball for the reasons discussed before.
2.
Switch off and disconnect the coil. Remove the aluminum ball. Properly fix the small, thin wire at the
threaded end of the terminal, such that it is pointing vertically upward.
3.
Again connect the coil and start the supply. A highly intense discharge can be observed to be emanating
from the tip of the wire. Such a discharge is called corona discharge.
From this demonstration, it can be deduced that the conductors with sharp pointed ends evolve high
frequency high voltage electric current than conductors having round, uniform and smooth surfaces. Hence
sharp edges and bends, pointed projections etc., are not used along with the high frequency electrical
instruments.
4.
Remove the metal wire after unplugging the coil and replace the aluminum ball terminal.
5.
Hold a small piece of plastic directly in contact with the aluminum ball terminal and start operating the coil.
6.
With the other hand bring a metal piece near the plastic. As it gradually nears the plastic piece, a discharge
will be observed through the plastic piece onto the metal surface. Hence it is implied that the high
frequency high voltage electric current passes through plastic onto the metal part. From this observation, it
can be concluded that the plastic, which is a very good insulator for normal low frequency electric current,
breaks down on the application of high frequency high voltage electric current and allows it to pass
through.
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STUDY OF IONIZATION OF GASES BY HIGH FREQUENCY AND HIGH VOLTAGE ELECTRIC
CURRENT
Additional items required:
(a)
(b)
(c)
(d)
Neon bulb with holder
Ordinary electric bulb
Fluorescent lamp tube
Thin conducting wire
Procedure:
1.
Take out the aluminum ball from the discharge terminal and thread the neon bulb with holder in it.
2.
Connect the coil to the power supply and start operating it. A very fine and luminous glow is observed
inside the neon lamp, although there is only a single connection between the coil and neon bulb.
3.
On touching the surface of the bulb with a metal wire, the brightness of the discharge increases and is
concentrated on a point, where the metal wire is touching the bulb.
This happens due to the ionization of gases inside the bulb. Gases get ionized due to very high voltage,
which knocks off the electrons from the atoms and is associated with the liberation of energy in the form of
light. The color of the glow varies for different gases and is the unique property of each gas.
4.
Take out the neon bulb with holder after disconnecting the coil and replace it with the aluminum ball.
5.
Again, plug in the coil and start operating it.
6.
Take the ordinary electric bulb and gradually bring it near the aluminum ball such that the metallic base of
the bulb is towards the aluminum ball.
7.
When the bulb is adequately close to the aluminum ball, it will start glowing thus showing that the gas
contained in the bulb has ionized whereas, there is no ionization in the air surrounding the aluminum ball.
The electric stress due to output of the coil is more at the surface of the aluminum ball as compared to the
point when the bulb begins to glow. The main reason behind it is that the gases contained in the bulb are at
a lower pressure than the air surrounding the aluminum ball (which is at atmospheric pressure).
From this experiment, it can be deduced that the pressure affects the ionization of gases and the lower the
pressure, more easily a gas will get ionized.
8.
Repeat step (6) and (7) with the neon bulb also and note the distances in each case between aluminum ball
terminal and the bottom of the bulb where it just begins to glow.
It is observed that the ordinary bulb has to be moved closer to the aluminum ball than the neon bulb for the
discharge to begin (or the ionization to take place). The ordinary bulb contains nitrogen and the neon bulb
contains neon. It implies that nitrogen requires more electrical stress than neon to get ionized.
Hence, it is inferred that the nature of gases also influences their ionization. Certain gases like helium, neon,
argon, krypton, xenon etc., (which are inert gases) ionize more quickly and easily as compared to other gases.
SPARKING POTENTIAL
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Additional Items required:
Conducting wire 8” (wire should be stiff)
Procedure:
1.
In the tesla coil apparatus, connect a metal wire to the Bakelite knob of terminal provided at the base of the
coil. Please make sure that the wire is secured tightly and stands vertically erect after it is fixed in the
terminal.
2.
Connect the tesla coil and start operating it.
3.
Gradually move the top end of the wire, gradually, towards the aluminum ball.
4.
As the distance is sufficiently close, a spark will be observed across the top of the wire and aluminum ball.
At this stage, measure the distance between the two.
This is the distance over which the electric potential generated by the coil ionizes the air. The electrical
potential required for just inducing a spark is referred to as sparking potential. Under standard conditions
the sparking potential required for dry air is approximately 30000V per centimeter of the discharge.
If s is the distance measured (in cms.) at which a spark is just produced across the wire and the aluminum
ball as observed above, then the sparking potential between aluminum and the wire is:
= 30000 x s volts
This is the approximate value of voltage produced by the tesla coil.
TRANSMISSION OF HIGH VOLTAGE POWER
Items required: Stiff conducting wires (one of 12” length and other of 8” length)
Procedure:
•
Perform the experiment in comparatively darker room for better results.
1.
Connect the metal wire 12” long (as in the previous experiment) to the Bakelite knob at the base of the coil.
Ensure that it is secured tightly and stands vertically erect.
2.
Connect the other wire 8” long to the threaded terminal on top of corona cap, in such a way that both the
wires are parallel to each other.
3.
On operating the coil, the corona discharge is observed on both the wires.
4.
Slowly and carefully, decrease the distance between the two wires by moving them from their free ends. As
the distance is sufficiently close, the formation of sparks is observed resulting from the ionization of the air.
The distance between the two wires is to be maintained just close enough so that there is no spark
formation between the wires.
© 2006 Arbor Scientific * P.O. Box 2750, Ann Arbor, MI 48106-2750 * (800) 367-6695 * ww.arborsci.com
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This situation is analogous to the two transmission lines, which are at a considerable distance away from
each other and are parallel. In the similar way, these lines are capable of transmitting high voltages.
STUDY OF HIGH VOLTAGE BRUSH DISCHARGES
Items required:
(a) Ordinary light bulb with holder
(b) 12” long insulated multi core copper wire
Procedure:
1.
Attach bulb holder on top of the threaded terminal and fit bulb in it.
2.
Connect the tesla coil and start operating it.
3.
Rotate the buzzer knob anticlockwise to position where it just stops working.
4.
Gradually and carefully start turning the buzzer knob clockwise.
5.
Different types of discharges are observed inside the bulb for different positions of the knob and correspond
to the different voltage outputs of the coil. These results are more pronounced as the pressure of the gas
decrease.
6.
Take out the bulb and bulb holder and fix one stripped end of the insulated wire to the terminals and
position it in such a way that the other stripped end is pointing vertically upward. Spread out the different
strands of wire in the shape of a comb.
7.
Repeat the step (3) and (4) and observe the different types of discharge at the tip of each strand of coil for
different buzzer knob settings.
STUDY OF THE EFFECT OF FARADAY SHIELD
Items required:
(a) Neon lamp
(b) Metal foil sheet – 9” sq.
Procedure:
Faraday shield is a series of parallel conductors, each connected along one common end (arranged in the fashion
of a comb) and the common connection is grounded. It is used for electrostatic shielding in such a way that the
electric field is not disturbed.
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1.
Cut slots in the piece of metal foil, in such a way that the slots remain joined at one end. This is a simple
faraday shield.
2.
Operate the tesla coil with aluminum ball terminal.
3.
Gradually bring the neon bulb near the aluminum ball by holding it from the glass portion such that its
bottom (metallic) portion is toward the aluminum ball.
4.
As the distance is close enough, the discharge is observed in the bulb. Stop at this point.
5.
Gradually, bring the Faraday Shield between the neon lamp and the aluminum ball by holding it in your
other hand. It is observed that the discharge disappears in the neon lamp when Faraday Shield is completely
between the lamp and the aluminum ball. This is due to the deionization of the neon gas inside the lamp,
since the Faraday Shield cuts off the electrostatic charge.
DEMONSTRATION OF ELECTROMAGNETIC FIELD
Items required:
(a)
(b)
(c)
(d)
Miniature flashlight bulb with screw base
Fluorescent tube
Aluminum disc
Multi core insulated copper wire – 9”
Procedure:
The high frequency electrical field consists of electrostatic radiation field and electro magnetic induction field,
the presence of which can be demonstrated.
1.
Instead of aluminum ball terminal, fix the aluminum plate on the discharge terminal of the coil on top of the
corona cup with the help of a nut.
2.
Operate the coil and gradually bring the metal base of the fluorescent tube near the aluminum plate (which
will act as an antenna).
3.
When the metal base of the fluorescent tube is adequately close to the aluminum plate, the gas inside the
tube will get ionized.
4.
Move the tube very slowly, away from the aluminum plate at this stage, the gas inside the tube is observed
to maintain its ionization. It will remain ionized even when the distance between the tube and the aluminum
plate is more than the distance at which the ionization begin (i.e., at a place where electrostatic stress is less
than that required for ionization). This is because the electrostatic stress, although not adequate to initiate
the process of ionization, is enough to maintain the ionization. This is due to the effect of the electrostatic
radiation field. This field can ionize a gas but cannot light the filament in a bulb.
5.
Unplug the apparatus and fix the aluminum ball terminal after removing the aluminum antenna plate.
6.
Take 9” length of multi core insulated copper wire and remove insulation from both of its ends. Bend it in
the form of a loop and fasten its ends to each screw terminal of the miniature flashlight bulb. Adjust the
© 2006 Arbor Scientific * P.O. Box 2750, Ann Arbor, MI 48106-2750 * (800) 367-6695 * ww.arborsci.com
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7.
loop around the corona cap underneath the aluminum ball terminal. In this way, the wire loop short-circuits
the miniature flashlight bulb. It nullifies any form of electrical effect due to the presence of electrostatic
radiation field (which is very feeble in nature).
In this case, the flashlight bulb is observed to glow very brightly. It is due to the presence of the another
field present in the electric field of tesla coil i.e., electromagnetic induction field.
TRANSMISSION OF HIGH FREQUENCY ELECTRICITY OVER A SINGLE CONDUCTOR
Items required:
(a) Ordinary light bulb (around 200 W) with holder
(b) Glass beaker
(c) Multi core insulated copper wire – 4 ft.
Procedure:
1.
Remove the insulation from both sides of the multi core insulated copper wire.
2.
Connect one end of the wire on the threaded discharge terminal below aluminum ball.
3.
Attach other end of the wire to the bulb holder and extend the wire to its full length.
4.
Insert bulb inside the bulb holder and place it inside the glass beaker (for properly insulating it from the
worktable on which tesla coil apparatus is placed).
5.
Operate the tesla coil and the bulb is observed to glow brightly.
This is because the high voltage high frequency electricity travels to the bulb through the connecting wire.
On reaching the light bulb, it emanates out of the bulb filament and ionizes the gas inside the bulb and
subsequently ionizing the air from which it travels back to the apparatus. In this case, air serves as the
second connection for the return path across them. Thus, high frequency electricity can be transmitted over
a single conductor.
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(a) Diagram 1: Tesla Coil
Short wire
Al Ball Terminal
Discharge Terminal
Corona Cap
Long Wire
Secondary Coil
Primary Coil
Bakelite Terminal
Capacitor
Spark Gap
Knob
Chord
Vibrator
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(b) Diagram 2: Schematic representation of Tesla Coil
Cp
P
S
G
E
Induction Coil
Ball Terminal
Secondary Coil
Capacitors 0.047 µF each
Spark Gap
Primary Coil
Iron
Core
Grounded Plug
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