experiment of metre bridge

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Meter Bridge Notes

Meter Bridge: A meter bridge, also known as a slide wire bridge, is a device that works on the Wheatstone bridge idea. A metre bridge is used to find the unknown resistance of a conductor. In Physics, while theory forms the basis of our knowledge, practicals form our understanding. What is this device? This is a   meter bridge! It consists of a wire of one meter, which is why it is called a ‘meter bridge’. It is used to measure the resistance of wires, coils or any other material. Please read on to learn about the meter bridge formula, meter bridge diagram, and more.

In this article, we will provide you with all information about the meter Bridge, the principle of the meter bridge, meter bridge experiment class 12 etc, continue to learn the concept thoroughly and make no mistakes while answering questions on the meter bridge.

What is Meter Bridge?

A meter bridge is an electrical apparatus using which we can measure the value of unknown resistance. It is made using a metre long wire of uniform cross-section. This wire is either nichrome or manganin or constantan because they offer high resistance and low-temperature coefficient of resistance.

A meter bridge or Slide wire bridge is designed from a Wheatstone bridge. It is the most basic and functional application of a Wheatstone bridge.

The Principle of Meter Bridge

A meter bridge works on the principle of a Wheatstone bridge. A Wheatstone bridge is based on the principle of null deflection, i.e. when the ratio of resistances in the two arms is equal, no current will flow through the middle arm of the circuit. Consider the diagram of the Wheatstone bridge as shown below. It consists of four resistances \(P,\,Q,\,R\) and \(S\) with a battery of EMF \(E\).

In the balanced condition, no current flows through the galvanometer, and terminals, \(B\) and \(D\) are at the same potential. This condition arises when, \(\frac{P}{Q} = \frac{R}{S}\)

Construction of a Meter Bridge

• A meter bridge has a \(1\,\rm{m}\) long wire of uniform cross-section area, which is stretched tight.
• Between two metal strips that are bent at right angles, this wire is then carefully clamped, as shown in the diagram below:
• Within the gap between the metal stripes, resistances are connected. In the first gap, \(R\), a resistance box, and in the second gap, a small resistor wire \(S\) is connected.
• The endpoints within which the wire is clamped are connected to a key through the cell.
• A galvanometer is connected to the metallic right in the middle of the two gaps.
• A jockey is connected at the other end of the galvanometer (Here, a jockey is a metal rod with a knife-like edge at one end that slides over the potentiometer wire to make an electrical connection). The jockey is slid over the meter bridge wire till the galvanometer shows zero deflection.

Meter Bridge Working

• To begin with, move the jockey to the endpoints of the wire, i.e., \(A\) and \(C\). The deflection of the galvanometer should be opposite on both ends.
• From side \(A\), start sliding the jockey slowly over the wire and carefully observe where the deflection of the galvanometer comes out to be zero.
• If such a point is not obtained, try varying the resistance across the bridge by changing the resistance on the variable resistance.
• Slide the jockey over the wire and carefully observe the point on the wire where the deflection of the galvanometer comes out to be zero. This is the null point as represented by the point ‘\(B\)’ in the diagram.
• Obtain the length of the null point using the meter scale attached along the wire. This is the ‘balancing length’ of the meter bridge.
• Let the distance between points \(A\) and \(B\) be ‘\(l_1\)’.
• Let the distance between points \(B\) and \(C\) be ‘\(l_2\)’, where \(l_2 = 100 – l_1.\)

When the galvanometer shows null deflection, the meter bridge behaves like a Wheatstone bridge and can be represented as:

Find the Resistance of the Given Wire by Metre Bridge

If \(S\) is the unknown resistance in the above circuit, we can calculate its value using the meter bridge. In the balanced condition, \(\frac{R}{{\text{Resistance}}\;{\text{across}}\;{\text{length}}\;AB} = \frac{S}{{\text{Resistance}}\;{\text{across}}\;{\text{length}}\;BC}\) We know that the resistance \(r\) of a wire of length \(l\), area of cross-section \(A\) and resistivity \(ρ\) is given as, \(r = \frac{{\rho l}}{A}.\) Using this relation, if \(ρ\) be the resistivity and \(A\) be the area of cross-section of the given meter bridge wire, then the resistance across length \(AB = \frac{{\rho l_1}}{A}.\) The resistance across length \(BC = \frac{{\rho l_2}}{A}.\) Substituting these values in the above relation, we get: \(\frac{R}{{\frac{{\rho {l_1}}}{A}}} = \frac{S}{{\frac{{\rho {l_2}}}{A}}}\) or, \(\frac{R}{{{l_1}}} = \frac{S}{{{l_2}}}\) \(\frac{R}{{{l_1}}} = \frac{S}{{100 – {l_1}}}\) Thus, the unknown resistance, \(S = \left( {100 – {l_1}} \right)\frac{R}{{{l_1}}}\) We can calculate the specific resistivity of the unknown resistance by using the formula, \(\rho = \frac{{\pi {d^2}S}}{{4L}}\) Where \(d\) is the diameter of the wire, \(S\) is the unknown resistance (of the wire), and \(L\) is the length of the wire.

Meter Bridge Experiment Class 12

Equipment Required

1. Meter Bridge 2. Galvanometer 3. Connecting wires 4. Unknown resistance 5. Resistance Box 6. Jockey 7. One-way key 8. Screw Gauge 9. Lechlanche cell

1. Collect all the required instruments and make all the necessary connections, as demonstrated in the above figure. 2. Take some appropriate kind of resistance out from the resistance box ‘\(R\)’. 3. Now, place the jockey at point \(A\); look that there is a deflection within the galvanometer. When the jockey is moved from point \(A\) to Point \(C\), the deflection of the galvanometer must go from one side to the other side. If it is not observed, adjust the known resistance value. 4. Start sliding the jockey from \(A\) towards \(C\) and obtain the point where the deflection of the galvanometer is zero. 5. Proceed with the above strategy for various values of the ‘\(R\)’. Note probably around \(5 -10\) readings. 6. The point where the galvanometer gives null deflection is the balance point of the meter bridge for the given unknown resistance. 7. Measure the distance between point \(A\) and the balance point of a given wire using an ordinary meter scale and the radius of the wire using a screw gauge (Take at least five readings for both the quantities). 8. Compute the mean value of the unknown resistances obtained above. It will be equal to the sum of all the values of resistance divided by the total number of readings taken.

Errors in the Meter Bridge

The most common error that can affect the measurement accuracy of a meter bridge is the end error . The end error can come up due to the following reasons:

1. We know that along the length of the bridge wire, a scale is provided. If the zero of the scale does not coincide with the starting point of the bridge wire, the \(100\,\rm{cm}\) mark on the scale will not coincide with the endpoint of the wire. This will lead to incorrect measurements of the balancing length. 2. The non-uniformity of the metal wire might lead to the generation of stray resistance, and it will create an end error.

We can minimize end error by taking multiple readings of the experiment by interchanging the unknown and known resistance in the circuit and by calculating the final value of resistance by taking the mean of all the observations.

Solved Examples of Meter Bridge

Q.1. In a meter bridge, there are two unknown resistance \(R\) and \(S\). Find the ratio of \(R\) and \(S\) if the galvanometer shows a null deflection at \(20\,\rm{cm}\) from one end? Ans: The null deflection in the galvanometer is obtained at \(20\,\rm{cm}\) from one end. Let, \(L_1 = 20\,\rm{cm}\) So, \(L_2 = 100 – 20 = 80\,\rm{cm}\) Thus, the ratio of unknown resistance will be: \(\frac{R}{{{L_1}}} = \frac{S}{{{L_2}}}\) Thus, \(\frac{R}{{{S}}} = \frac{1}{{{4}}}\)

Q.2. A \(20\,\rm{Ω}\) resistor is connected in the left gap, and an unknown resistance is joined in the right gap of the meter bridge. Also, the null deflection point is shifted by \(40\,\rm{cm}\) when the resistors are interchanged. Find the value of unknown resistance? Ans: In the first case, let the deflection point is taken as \(L\). Let the balance point gets shifted to \(l\) by \(40\,\rm{cm}\) when the resistors are interchanged. Thus, \(L – l = 40\,\rm{cm}\) Also, \(L + l = 100\,\rm{cm}\) Solving the above equations, we get: \(l = 30\,\rm{cm}\) \(L = 70\,\rm{cm}\) Let \(R = 20\,\rm{Ω}\) And unknown resistance be \(S\), thus, \(\frac{R}{S} = \frac{L}{l}\) \(\frac{R}{S} = \frac{70}{30}\) \(\frac{20}{S} = \frac{7}{3}\) \(∴ S = 8.57\,\rm{Ω}\)

A meter bridge is an electrical apparatus using which we can measure the value of unknown resistance. It is made using a metre long wire of uniform cross-section. This wire is either nichrome or manganin or constantan. The principle of working of a meter bridge is the same as the principle of a Wheatstone bridge. A Wheatstone bridge is based on the principle of null deflection. Thus, the unknown resistance, \(S = \left( {100 – {l_1}} \right)\frac{R}{{{l_1}}}.\)

FAQs on Meter Bridge

Q.1: What is the principle of meter bridge? Ans: Meter bridge is based on the principle of the Wheatstone bridge.

Q.2: Why do we use constantan or manganin wire in a meter bridge? Ans: Constantan, manganin, or nichrome wires provide a low-temperature coefficient of resistance, so they are used in a meter bridge.

Q.3: What is a meter bridge, and what is it used for? Ans: A meter bridge is an electrical apparatus that is used to measure the unknown resistance of a conductor. It consists of a wire of length of one meter. Hence it is called a meter bridge.

Q.4: What is the end error in a meter bridge? Ans: End error occurs when the zero scales of the meter scale do not coincide with the starting of the wire. It is caused due to the shifting of zero scale or the stray resistance in the wire.

Q.5: Give the formula to measure the unknown resistance for a meter bridge. Ans: The unknown resistance, \(S = \left( {100 – {l_1}} \right)\frac{R}{{{l_1}}}\), where \(R\) is known resistance and \(l_1\) is the balancing length of the wire.

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To verify the laws of combination (series & parallel) of resistances using meter bridge (slide Wire Bridge)

A meter bridge, laclanche cell, a galvanometer, a resistance box, a jockey, two resistances wires, set square, sand paper and connecting wires.

Observations: Table for length (ι) & unknown resistance (r):

Calculations:

In Series: Experimental value of R S = 2.72Ω

Theoretical value of R S =r 1 + r 2 = 2.75Ω

In parallel: Experimental value of R P =0.66Ω

Theoretical value of R P =r 1 r 2 /r 1 + r 2 = 0.68Ω

Result: Within limits of experimental error, experimental & theoretical values of R S are same. Hence the law of resistance in series i.e. R s = r 1 + r 2 is verified. (1) Within limits of experimental error, experimental & theoretical values of R p are same. Hence law of resistances in parallel i.e. R s = r 1 r 2 /r 1 +r 2 is verified.

Precautions:

• The connections should be neat, clean & tight.
• Move the jockey gently over the wire & don’t rub it.
• All plugs in resistant box should be tight.

Sources of Error:

• The plugs may not be clean.
• The instrument screws maybe loose.

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Class 12 Physics Lab Experiment list

• 1 To find resistance of a given wire using Whetstone’s bridge (meter bridge)
• 2 To find the focal length of a convex mirror using a convex lens
• 3 To find the value of ‘v’ for different values of ‘u’ in case of a concave mirror & to find its focal length
• 4 To draw the characteristics curves of a zener diode vs to determine its reverse breakdown voltage
• 5 To find the focal length of a concave lens using a convex lens
• 6 To determine angle of minimum deviation for a given prism
• 7 To determine the refractive index of a glass using travelling microscope

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Meter Bridge Resistance of a Wire

Physics Practicals Class 12

Meter Bridge – Resistance of a Wire

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About Simulation

• In this simulation, you will correlate the principle of Wheatstone bridge with the meter bridge experiment for physics practical class 12.
• You will learn the theory behind Wheatstone’s meter bridge and examine the resistance of a wire.
• You will determine the resistivity (specific resistance) of a given material of the wire.
• All the experiment steps and procedures, such as connecting the wire, measuring the balancing lengths, observing the null deflection of the galvanometer, etc., are highly interactive and have been precisely recreated in a manner that is very similar to what you would do in a real lab.

• This interaction provides a very immersive virtual reality environment and gives you a real-lab-like experience while conducting or performing experiments.

Simulation Details

Description

The meter bridge, also known as the slide wire bridge consists of a 1-meter-long wire of uniform cross-sectional area, fixed on a wooden block. A scale is attached to the block. Two gaps are formed on it by using thick metal strips in order to make the Wheatstone bridge.

The meter bridge operates using the Wheatstone principle. Here, four resistors P, Q, R, and S are connected to form the network ABCD. Terminals A and C are connected to a battery, and the terminals B and D are connected to a galvanometer.

In the balancing condition, there is no deflection on the galvanometer. Then, $$\frac{P}{Q}=\frac{R}{S}$$

If a resistance wire of unknown resistance 𝑋 is introduced in the right gap of the meter bridge and the high resistance 𝑅 is introduced in the left gap of the meter bridge, then as the jockey slides over the bridge wire, it shows zero deflection at the balancing point (null point).

If the balancing length is 𝑙, then according to the Wheatstone principle, we have $$\frac{X}{R}=\frac{l}{100-l}$$

The unknown resistance is given by $$X=R \frac{l}{100-l}$$

The specific resistance of the wire can be calculated using the relation, $$\rho=\frac{\pi r^2 X}{L}$$

where, 𝐿 is the length of the wire and 𝑟 is its radius.

Requirements for this Science Experiment

⦁ Meter Bridge ⦁ Jockey ⦁ Resistance Box ⦁ Plug Key ⦁ Battery

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Notes For Free

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Finding the resistance of a given wire using meter bridge and hence determine resistivity of its material

Aim – To find the resistance of a given wire using meter bridge and hence determine resistivity of its material .

A meter bridge (slide wire bridge ) , a lecranche cell, a galvanometer , a resistor , a jockey , a one way key , a resistance wire , a screw gauge , a meter scale , a set of square , connecting wire , a piece of sand paper .

The unknown resistance ‘X” is given by

• Where ‘R” is the known resistance placed in the left gap & unknown resistance ‘X” is the right gap of meter bridge .’L’ is length of meter bridge wire from zero end upto balance .
• Resistivity of the material of the given wire is given by

Where ‘L’ is the length & D is the diameter of the given wire .

• Arrange the apparatus as shown in the arrangement diagram .
• Connect the resistance wire whose resistance is to be determine in the right gap b/w C& B .Take care that no point /part of the wire forms a loop.
• Connect resistance box of low range in the left gap b/w A & B .
• Make all other connection as shown in the circuit diagram .
• Take out some resistance from the resistance box , ping the key ‘K’
• Touch the jockey gently first at length end & then right end of the bridge wire .
• Note the deflection in the galvanometer. If the galvanometer shows deflection in the galvanometer reading in opposite direction the correction are correct. If the deflection is on one side only then there is fault in the circuit . Check & rectify the fault .
• Move the jockey gently along the wire from left to right till gives zero deflection . The point where the jockey is touching the wire is null point ‘D’.
• Choose an appropriate value of ‘R’ from the box such that there id no defection in the galvanometer when the jockey is nearly in the middle of the wire .
• Note position of point ‘D’ to known the length AD=l
• Take atleast 4 sets of observation in the same way by changing the value of R in the steps .
• Record your observation .
• Cut the resistance wire at the point where it leaves the terminal , started it & find its length by using a meter scale .
• Measure the diameter of the wire at least at 4 places in two mutually perpendicular direction at each place with the of screw gauge .
• Record your observation as given in the table .

Observation

• Length of a given wire L = 66 cm =0.66 m
• Table for unknown resistance (X)

3. Least count of screw gauge

Pitch of screw gauge =0.01

Total no of division on the circular scale =

LC of screw gauge = Pitch /No of the circular scale

Zero error (e) =(0)

Zero connection =(e)=0

Radius of the resistance wire

• The value of the unknown resistance X =0.5 ohm
• The specific resistance of material of wire = 0.104 x10 -3 ohm m
• Percentage error

The connection should be neat, clean & tight.

Source of error

Plug may not be clean

The wire may not be of uniform thickness.

Viva questions

• Why is the meter bridge so called?

Since the bridge uses one meter long wire, it is called a meter bridge .

• What is a null point ?

It is a point on the wire , keeping jockey at which galvanometer gives 0 deflection .

• Why is the bridge method better that ohms law for measurement ?

It is so because the bridge method is a null method (at null point , no current is flowing in galvanometer ) and more sensitive .

• Why copper strips used to press ends of wire are thick ?

Thick Cu strips have negligible resistance over the resistance of alloy meter bridge wire and minimize affect of end resistance .

Circuit diagram

You can also get Class XII Practicals on  Biology ,  Physics , and  Physical Education .

28 Replies to “Finding the resistance of a given wire using meter bridge and hence determine resistivity of its material”

hiii sir i am sushant raj from cbse i want to help in to my gmail physics pratical ..

Hii myself trisha singh of st thomas school,ranchi of isc board heartly grateful to u for ur best notes of exp related to phy practical.

This is very good notes for intermediate students

standard value of the specific resistance of the material of the given. what?

Thanks guru

So gud notes sir its is so helpful to us

Its very helpful sir

It was very helpful

What is the material of wire used?? Please specify…..

the material of wire used in meter bridge is made of magnanin.

meter bridge experiment discussion ???

The wire should have been of constantun as most of the time it is. But the result doesn’t match. Probably it is because of the experimental errors. It can be possible that he has taken a wire of different material. No need to worry as these problems would vanish once you perform the experiment yourself.

I want to know that when ‘ R’ is in left gap and when R is in right gap

gjb notes good carryon

Thankuu so much. It helped me a lot for my practical experiment..I also recommended this site to my friends.. Wil you plz put up some questions of physics Sec -B expermt Nd full experiment also .

Why 4 is not used in the final formula?

It is not 0.21 ×10^-4 m It is 0.21×10^-3 m

***** 5 star

But sir, what about the conclusion for the experiment

Awsome guru thanks for uploading this data it was really healful to me

good day sir pls I need an answer to this:a battery of 1.50v has a terminal p.d of 1.25v when a resistor of 25ohms is joined to it.calculate the current flowing,the internal resistance r and the terminal p.d when a resistor of 10 ohms replaces the 25 ohms resistor. Thanks

Specific resistance obesarvation

Really these notes were very helpful to me actually I was finding correct reading and calculation of practical but I found no sites which has given correct reading of the practical. I really appreciated the work of creator who has created this content in this website.

Must Mari bhot help ho Gai In physics ka practical ma Thanks

This is really helpful, thank you for this great work.

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Experiment - to find resistivity of the material of a given wire using metre-bridge. - all with video answers.

Chapter Questions

Problem 2845

In meter bridge experiment, A thin uniform wire $\mathrm{AB}$ of length $1 \mathrm{~m}$ and unknown resistance $\mathrm{x}$ and a resistance of $12 \Omega$ are connected. In the above question, after appropriate conditions are made, it is found that no deflection takes places in the galvanometer when the sliding jockey touches the wire at a distance of $60 \mathrm{~cm}$ from $\mathrm{A}$. What is the value of the resistance $\mathrm{X}$ ? (A) $18 \Omega$ (B) $8 \Omega$ (C) $16 \Omega$ (D) $4 \Omega$

Problem 2846

A thin uniform wire $\mathrm{AB}$ of length $1 \mathrm{~m}$, an unknown resistance $\mathrm{X}$ and a resistance of $12 \Omega$ are connected by thick conducting strips as shown in figure. A battery and a galvanometer (with a sliding jockey connected to it) are also available. Connections are to measure the unknown resistance $\mathrm{X}$ using the principle of Wheatstone bridge. The appropriate connections are. (E Is the balance point for Wheatstone bridge) (A) battery across $E B$ and galvanometer across $B C$ (B) battery across $\mathrm{EC}$ and galvanometer across $\mathrm{BD}$ (C) battery across $\mathrm{BD}$ and galvanometer across $\mathrm{EC}$ (D) battery across $\mathrm{BC}$ and galvanometer across $\mathrm{CD}$

Problem 2847

A wire is in the form of a tetrahedron shown in figure. The resistance of each wire is $\mathrm{R}$. What is the resistance of the frame between the corners $\mathrm{A}$ and $\mathrm{B}$. (A) $(2 \mathrm{R} / 3)$ (B) $2 \mathrm{R}$ (C) $\mathrm{R}$ (D) $(\mathrm{R} / 2)$

Problem 2848

For the electrical circuit shown in the figure, the potential difference across the resistor of $400 \Omega$ as will be measured by the voltmeter $\mathrm{V}$ of resistance 400 is $\ldots \ldots \ldots$ (A) $(10 / 3) \mathrm{V}$ (B) $4 \mathrm{~V}$ (C) $(20 / 3) \mathrm{V}$ (D) $5 \mathrm{~V}$

Problem 2849

In a simple meter-bridge circuit, the both gaps are bridge by coils $\mathrm{P}$ and $\mathrm{Q}$ having the smaller resistance. A balance is obtained when the jockey key makes contact at a point of the bridge wire $40 \mathrm{~cm}$ from the $\mathrm{P}$ end. On shunting the coil $\mathrm{Q}$ with a resistance of $50 \Omega$ the balance point is moved through $10 \mathrm{~cm}$. What are the resistance of $\mathrm{P}$ and $\mathrm{Q}$ ? (A) $[(100) / 3] \Omega,[(100) / 2] \Omega$ respectively (B) $[(50) / 3] \Omega,[(50) / 2] \Omega$ respectively (C) $[(25) / 3] \Omega,[(25) / 2] \Omega$ respectively (D) $[(75) / 3] \Omega,[(75) / 2] \Omega$ respectively

Problem 2850

What is the resistance of an open key? (A) $\infty$ (B) Can't be determined (C) 0 (D) depends on the other resistance in the circuit

Problem 2851

What is the unit of temperature coefficient of resistance? (A) $\Omega^{-1}{ }^{\circ} \mathrm{C}$ (B) $\Omega^{1}{ }^{\circ} \mathrm{C}^{-1}$ (C) ${ }^{\circ} \mathrm{C}^{-1}$ (D) $\Omega^{0}{ }^{\circ} \mathrm{C}^{-1}$

• Physics Article
• To Verify The Laws Of Parallel Combination Of Resistances Using A Metre Bridge Experiment

To Verify The Laws Of Combination (Parallel) Of Resistances Using A Metre Bridge

To verify the laws of combination (parallel) of resistances using a metre bridge

Materials Required:

1. 2 different resistances (carbon or wire-wound resistors), 2. metre bridge, 3. galvanometer, 4. a cell or battery eliminator, 5. a jockey, 6. a rheostat, 7. a resistance box, 8. a plug key, 9. sandpaper and 10. thick connecting wires.

Consider two resistances, r1 and r2, are connected in series.

The series combination resistance RS is given by RS = r1 + r2

When connected in parallel, the resistance of the combination is given by Rp

Resistance connected in Parallel

Resistances r1 and r2 connected in parallel to one arm of a metre bridge

• Set up the circuit as shown in the figure above.
• Connect R1 and R2 as shown in the figure.
• Tighten all plugs in the resistance box by pressing and rotating each plug to assure that all plugs make good electrical connections. Using sandpaper, clean the ends of connecting wires before making the connections.
• Remove some plug(s) from the resistance box to get the suitable value of resistance R
• Get a null point D on the metre bridge wire by sliding the jockey between ends A and C.
• Note the value of the resistance R and lengths AD and DC.
• Calculate the experimental value of the equivalent parallel resistance
• Repeat the experiment for four more values of resistance R. Obtain the mean value of unknown resistance.

Observations And Calculations

Precautions

• Ensure the connection is neat, clean and tight
• Insert key only while taking an observation
• Move jockey gently over the metre bridge wire.

Sources Of Error

• The instrument screw may be loose
• Unavailability of thick connecting wires
• The experimental value of Xp = ohm
• The theoretical value of Xp = ohm
• The experimental and theoretical value of Xp is found to be the same. Hence, the law of resistance in parallel is verified.

Viva Questions

1. List the factors affecting the resistance?

ANS: The factors affecting the resistance are,

• Area of cross-section
• The temperature of the conductors

2. What is the mathematical form of Ohm’s law?

3. What is resistance?

ANS: The ratio of potential difference V across the ends of a conductor to the current I flowing through it. It is represented by R and is given by R=V/I

4. What is ohmic resistance?

ANS: The resistance which obeys ohms law is known as Ohmic resistance.

5. Give examples of non-ohmic resistance?

ANS: Transistors, vacuum tube diodes and semiconductor diodes.

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• Wheatstone Bridge, Meter Bridge and Potentiometer

Every other day, science presents us with one or more ways to feel amazed. There are a host of experiments that show both how we can use things and make newer things out of them. Experiments related to Wheatstone Bridge and the potentiometer are among few such things in science that invoke a curious sense of amazement. Let us study more about the concept of Wheatstone bridge and meter bridge, along with potentiometer.

Suggested Videos

The concept of wheatstone bridge.

Defined simply, a Wheatstone Bridge is an electric circuit that is used to measure the electrical resistance of a circuit. The circuit is set out by balancing two legs of a bridge circuit. Out of the two, one of the legs is an unknown component which was invented by Samuel Hunter Christie in the year 1833 and later, it improved and popularized by Sir Charles Wheatstone in the year 1843.

Nowadays, technological progress has allowed us to make various measurements through sophisticated tools and machines. However, even today, the wheat bridge remains an authentic way to measure electric resistance, down to the closest milliohms as well.

Browse more Topics under Current Electricity

• Electric Current
• Electrical Energy and Power
• Resistivity of Various Materials
• Temperature Dependence of Resistivity
• Drift of Electrons and the Origin of Resistivity
• Combination of Resistors – Series and Parallel
• Atmospheric Electricity and Kirchhoff’s Law
• Cells, EMF, Internal Resistance
• Cells in Series and Parallel

You can download Current Electricity Sheet by clicking on the download button below

The Principle behind the Wheatstone Bridge

The usual arrangement of the Wheat stone bridge circuit has four arms. The bridge circuit where the arms are situated consist of electrical resistance. Out of these resistances, P and Q are the fixed electrical resistances and these two arms are the ratio arms. Next, A Galvanometer connects between the terminals B and D through a switch K 2 . The source of voltage of this arrangement is connected to the terminals A and C through a switch, K 1 .

A variable resistor S is connected between point C and D. The potential at point D is altered by adjusting the value of a variable resistor. If a variation in the electrical resistance value of arm CD is brought, the value of current I2 will also vary as the voltage across both A and C is fixed.

If we continue to adjust the variable resistance, a situation may come when the voltage drops across the resistor S that is I2. Here, S becomes exactly equal to the voltage drop across resistor Q that is I1. Q. So, the potential at point B becomes equal to the potential at point D hence the potential difference between these two points is zero hence current through galvanometer is nil. The deflection in the galvanometer is nil when the switch K2 is closed.

Applying Kirchoff’Law, in this condition,

How is the meter bridge experiment carried out using the wheatstone principle.

The meter bridge experiment uses the wheat bridge experiment to demonstrate the resistance of an unknown conductor or to make a comparison between two unknown resistors. Through the above-stated equation, one can easily decipher the specific resistance of a given material

Conclusions of the wheat stone bridge principle are:

According to the Wheatstone-bridge principle, the resistance of length AB/resistance of length BC = R / X

Let l be the length of wire between A and B and then (100 – l) is the length of wire between B and C. Here, P = ρl / A. Since the wire has a uniform cross-section and ρ is constant. Its resistance is proportional to the length. That is P ∝ l, and Q ∝ (100–l). So,

L / (100–l) = R / X

This is how to draw the values of X for different values of R and the mean value gives the value of unknown resistance X.

The Concept of Potentiometer

A potentiometer is an electric device which is used to regulate EMF and internal resistance of a given cell . This helps in providing a variable resistance and therefore, a variable potential difference arising between two points in an electric circuit. It is basically a three-terminal resistor device with an adjustable arm that increases or reduces the resistance in the loop.

Potentiometer (Source: Wikipedia)

Solved Examples for You

Question: Describe how a potentiometer works in an arrangement.

Answer: A potentiometer consists of a uniform wire AB of manganin or constantan that has a length of usually 10 m. it is kept stretched between copper stripes that are fixed on a wooden board by the side of a metre scale. The wire is then divided into ten segments each of 1 m length.

These segments join in series through metal strips between points A and B. A steady current is maintained in the wire AB by a constant source of EMF Eo, called driver cell, that connects between A and B through a rheostat. A jockey slides over the potentiometer wire which makes contact with the wire and cell.

Potentiometer (Source: Wikimedia)

Thus we can say that the potential difference across any portion of the potential of the potentiometer wire is directly proportional to the length of that portion provided the current is uniform.

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Current Electricity

• Electrical Insulators
• Electric Car
• Electrical Conductors
• Electric Power
• Cells, EMF and Internal Resistance
• Atmospheric Electricity and Kirchhoff’s Law
• Combination of Resistors in Series and Parallel

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