A Post Office Box can also be used to measure an unknown resistance. It is a Wheatstone Bridge with three arms P, Q and R; while the fourth arm is the unknown resistance. P and Q are known as the ratio arms while R is known as the rheostat arm.
At balance, the unknown resistance
S = (P/Q)R. . . (1)
The ratio arms are first adjusted so that they carry 100 Ω each. The resistance in the rheostat arm is now adjusted so that the galvanometer deflection is in one direction. If R = R0 (ohm) and R = R0 + 1 (ohm) are the resistance in rheostat arm, for which the deflection in galvanometer is in opposite direction, then it implies that the unknown resistance ‘S’ lies between R0 & (R0 + 1) ohm.
Now, the resistances in P and Q are made 100 Ω and 1000 Ω, respectively, and the process is repeated.
Equation (1) is used to compute S. The ratio P/Q is progressively made 1:10, and then 1:100. Thus, the resistance S can be accurately measured.
The major sources of error are the connecting wires, unclear resistance plugs, change in resistance due to Joule heating, and the insensitivity of the Wheatstone bridge.
These errors may be removed by using thick connecting wires, clean plugs, keeping the circuit on for very brief periods (to avoid Joule heating) and calculating the sensitivity.
In order that the sensitivity is maximum, the resistance in the arm P is kept close to the value of the resistance S.
Illustration14. In an experiment with a post−office box, the ratio arms are 1000 : 10. If the value of the third resistance is 999 Ω,. find the unknown resistance.
Solution: The ratio arms are 1000 : 10
∴ P/Q = 1000/10 = 100
Third resistance R = 999 Ω
Let x be the unknown resistance.
We know that,
∴ X = Q/P x R = 1/100 x 999 = 9.99 Ω
Illustration15.When two resistances X and Y are put in the left hand and right hand gaps in a meter bridge, the null point is at 60 cm. If X is shunted by a resistance equal to half of itself then find the shift in the null point.
Solution :Arrangement is shown in the figure. X/Y = 60/40 = 3/2 . . (1) When X is shunted, then resistance in the left gap becomes
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⇒I = 33.3 ∴ Shift = 60 – 33.3 = 26.7 cm
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A Post Office Box can also be used to measure an unknown resistance. It is a Wheatstone Bridge with three arms P, Q and R; while the fourth arm(s) is the unknown resistance. P and Q are known as the ratio arms while R is known as the rheostat arm.
At balance, the unknown resistance
S = (P/Q) R …… (1)
The ratio arms are first adjusted so that they carry 100 W each. The resistance in the rheostat arm is now adjusted so that the galvanometer deflection is in one direction. If R = R0 (ohm) and R = R0 + 1 (ohm) are the resistance in rheostat arm, for which the deflection in galvanometer is in opposite direction, then it implies that the unknown resistance ‘s’ lies between R0 & (R0 + 1) ohm.
Alt txt: resistance-using-post-office-box
Now, the resistance in P and Q are made 100 W and 1000 W, respectively, and the process is repeated. Equation (1) is used to compute S. The ratio P/Q is progressively made 1:10, and then 1:100. Thus, the resistance S can be accurately measured.
The major sources of error are the connecting wires, unclear resistance plugs, change in resistance due to Joule heating, and the insensitivity of the Wheatstone bridge
These errors may be removed by using thick connecting wires, clean plugs, keeping the circuit on for very brief periods (to avoid Joule heating) and calculating the sensitivity.
In order that the sensitivity is maximum, the resistance in the arm P is kept close to the value of the resistance S.
Illustration:
Resistance of a conductor is 1.72 W at a temperature of 20°C. Find the resistance at 0oC and 100°C. Given the coefficient of resistivity is a = 0.00393/oC.
R = R0 (1 + aDT)
R = 1.72 W (1 + 0.00393) × (0°C – 20°C) = 1.58 W
At T = 100°C
R = 1.72 W [1 + 0.00393) × (100°C – 20°C)] = 2.26 W
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The post office box was a Wheatstone bridge-style testing device with pegs and spring arms to close electrical circuits and measure properties of the circuit under test.
Table of Contents
The box containing Wheatstone bridge was used in the United Kingdom by engineers in General Post Office, who used it in UK telecommunications to trace electrical faults. Therefore, for historical reasons it was named “Post office box” as it was carried by post office engineers.
The best material for resistance in post box resistor is Manganin. It is an alloy of copper (85%), Manganese (12%) and Nickel (2%).
Post Office Box is helpful in physics experiment to determine the unknown resistance of wires like Constantan and Nichrome etc. With the help of Nvis 6041, the student can learn the principle of Post Office Box and its working. It can determine the specific resistance of different types of wire.
A post box is a numbered box which are available on rent at certain major post offices. Every post box is allocated a number and only postal articles bearing that number are placed into that box.
People mail their letters and packages by placing them in a mailbox or taking them directly to the post office. At the post office, postal workers sort the mail by size. Letters go through a machine that cancels the stamps. This means that the machine prints lines over the stamps so that they cannot be used again.
A Post Office Box is used to measure an unknown resistance. It is basically a Wheatstone bridge with three arms known as P, Q, and R; while the fourth arm is the unknown resistance, denoted by S.
ρ=Lπr2X ; Where L be the length of the wire and r be its radius.
What is the Wheatstone Bridge Principle? The Wheatstone bridge works on the principle of null deflection, i.e. the ratio of their resistances is equal, and no current flows through the circuit. Under normal conditions, the bridge is in an unbalanced condition where current flows through the galvanometer.
If a parcel contains cloth or woolen material it must be packed in a strong wrapper with an outer covering of stout card-board or cloth. It is suggested to use a wooden or a stout cardboard case according to the nature of the article for packing a parcel.
No, you cannot send food items through India Post.
D) Principle of wheatstone bridge. Hint: Post office box is a wheatstone bridge style testing device with pegs and spring arms in order to close the electrical circuit to measure the properties of the circuit under test. Basically, a post office box is a measuring device.
The half deflection method is the method by which an internal resistance of a cell can help in evaluating value which is considered as high. The deflection of half is considered as a value obtained along with the cell alone.
The potentiometer is an instrument used for measuring the unknown voltage by comparing it with the known voltage. It can be used to determine the emf and internal resistance of the given cell and also used to compare the emf of different cells. The comparative method is used by the potentiometer.
In early Victorian era the color of post box was Green in Britain. Then during the mid 19th century it was changed to Red. So with the Britishers the color of post box also came to its colonies like India, Australia, etc.
However, there is some variety indicating uncertainty as to the correct green to be used. Royal Mail have agreed that, only where there is historic precedent, will post boxes be painted in any colour other than the current official, standard colour- red.
Channel Islands Box (1852) The first post box installed by the British Post Office.
The primary function of Post Office is collection, processing, transmission and delivery of mail.
Postal services are also valuable to the government of some countries. They use the services to coordinate with other offices- to send notices, memos, and other documents. It is useful especially to places that are hard to reach. They event sent packages and parcels from one post office to another.
It is basically classified into 3 types, namely – Head Post office, Sub Post Office and Branch Post Office.
Potentionmeter measures the potential difference more accurately than a voltmeter, because the potentionmeter.
In a meter bridge experiment, a null point is obtained at 20 cm from one end of the wire when resistance x is balanced against another resistance y. If x In a metre bridge, wires of nichrome, manganin and constantan are used because it has high resistance and low-temperature coefficient of resistance. The jockey is used to slide on the bridge wire. It is a metal rod with one end as knife edge. Now adjust the value of resistance in the resistance box and slide the jockey along the wire. This process is to be done until the galvanometer shows a zero or null deflection. Craving More Content? What causes the standing waves on a violin. The energy of a vibrating string is transmitted through the bridge to the body of the violin, which allows the sound to radiate into the surrounding… The generic roller coaster vertical loop, where a section of track causes the riders to complete a 360 degree turn, is the most basic of roller… The resultant or net force will be = (100 – 80) N = 20 N. Q. In a game of tug of war, three girls of… To verify laws of resistances in series by post office box. To verify laws of resistances in series, a post office box is used to measure unknown resistance. It works on the principle of Wheatstone bridge . P, Q, R and S be the four resistances of the Wheatstone bridge as shown in figure 1. Here, P, Q and R are the known resistances and S is the unknown resistance. A galvanometer is connected between B and D. When the galvanometer current is zero (null condition ), the relation between P, Q, R and S is \frac{P}{Q} = \frac{R}{S} S= \frac{Q}{P}R Where, S is the equivalent resistance of two resistances connected in series. P and Q are the resistances in the ratio arm and R is the resistance of the rheostat arm. Or, S= r_{1}+r_{2}Which wire is used in meter bridge?
Why jockey is used in meter bridge?
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post office box experiment in physics | practical
Table for finding equivalent resistance in series connection
1. why is the instrument called p.o (post office ) box .
Ans : This instrument is used to measure the unknown resistances of electric cable and telegraphic wires. It is used by the postal department to find resistance of communication wires. This is why the name of this instrument is post office (P.O) box.
Ans: It works on the principle of Wheatstone bridge.
Ans: The condition at which the galvanometer deflection is zero is called null condition. At null condition, the relation between the four resistances P, Q, R and S is \frac{P}{Q} =\frac{R}{S}
Ans : See procedure no. (3)
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The resistance of a wire of length l and diameter D is given by \begin{align} R=\frac{\rho l}{A}=\frac{4\rho l }{\pi D^2},\nonumber \end{align} where $\rho$ is resistivity (or specific resistance) of the material of the wire. The unit of resistance R is Ohm ($\Omega$) and that of the resistivity is Ohm-metre ($\Omega\text{-m}$). The resistivity is a material property and it is reciprocal of conductivity.
To find the resistivity of the material of the wire, we need to measure length l by using a scale, diameter D by using a screw gauge , and resistance R by using a metre bridge. The metre bridge works on the principle of balanced Wheatstone bridge.
Metre bridge principle, procedural steps.
Question 1: In a metre bridge experiment, it is recommended to have known and unknown resistances of the similar values. This recommendation A. reduces error in finding position of the null point. B. makes null point to lie in the middle of the wire. C. improves sensitivity of the galvanometer. D. reduces current through the galvanometer.
Question 2: In a metre bridge experiment, two students have identical setup and follow similar procedure. But one of the student uses very low source voltage V . This student gets more error because A. it reduces sensitivity of the galvanometer. B. it increases error in position of the null point. C. it increases temperature of the wire. D. it reduces current in the wire.
Question 3: In a modified metre bridge, the length of the bridge wire is reduced to 50 cm from 100 cm. This will A. decrease the error in finding the null point. B. increase the error in finding the null point.
Question 4: In a metre bridge experiment, the known and the unknown resistances are interchanged to remove A. index error. B. end correction error. C. error due to temperature effect. D. random error.
Question 5: The bridge wire in metre bridge construction is made of materials like constantan or manganin because A. they have small resistivity and small temperature coefficient of resistivity. B. they have large resistivity and large temperature coefficient of resistivity. C. they have small resistivity and small temperature coefficient of resistivity. D. they have large resistivity and small temperature coefficient of resistivity.
Problem from iit jee 2014.
Solution: Let $\lambda$ be resistance per unit length (in ohm/cm) of the potentiometer wire. Total length of the wire is 100 cm and null point is obtained at $x=40\;\mathrm{cm}$. The resistances of four branches of Wheatstone bridge are, $R_1=R$, $R_2=90\;\Omega$, $R_3=\lambda x$, and $R_4=\lambda(100-x)$. The Wheatstone bridge is balanced if, \begin{align} &\frac{R_1}{R_2}=\frac{R_3}{R_4}, & &\text{or} & & \frac{R}{90}=\frac{x}{100-x}. \end{align} Solve to get $R=60\;\Omega$.
The least count of scale gives error in measurement of $x$, i.e., $\Delta x=0.1\;\mathrm{cm}$. To find error in $R$, differentiate above equation and simplify to get, \begin{alignat}{2} &\frac{\Delta R}{R}=\frac{\Delta x}{x}+\frac{\Delta x}{100-x}. \end{alignat} Substitute values and then solve to get $\Delta R=0.25\;\Omega$.
Statement 1: In a metre bridge experiment, null point for an unknown resistance is measured. Now, the unknown resistance is put inside an enclosure maintained at a higher temperature. The null point can be obtained at the same point as before by decreasing the value of the standard resistance.
Statement 2: Resistance of a metal increase with increase in temperature.
Solution: If radius of the wire AB is doubled then its resistance ($R=\rho l/A$) becomes one fourth and current ($i=V/R$) becomes four times. However, the potential drop per unit length, $V/l$, remains same.
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In this situation, unknown resistance S = R/10. (v) Similarly, 1000 ohm resistance is inserted in arm P and in Q arm 10-ohm resistance is inserted and the experiment is performed as before. Experiment: To determine the unknown resistance by a Post Office Box Theory: By inserting appropriate resistance in the post office box current passing.
A Post Office Box can also be used to measure an unknown resistance. It is a Wheatstone Bridge with three arms P, Q and R; while the fourth arm is the unknown resistance. P and Q are known as the ratio arms while R is known as the rheostat arm. At balance, the unknown resistance. S = (P/Q)R. . . (1)
The post office box is the another example of wheatstone bridge principle. In this instrument all the three unknown resistances P, Q and R are suitably arranged in a resistance box of special design as shown in Fig. 23.24. The unknown resistance forms the fourth arm of the bridge. In the top row of the instrument, there are six resistances in ...
Alt txt: resistance-using-post-office-box. Now, the resistance in P and Q are made 100 W and 1000 W, respectively, and the process is repeated. Equation (1) is used to compute S. The ratio P/Q is progressively made 1:10, and then 1:100. Thus, the resistance S can be accurately measured. Errors
Post Office Box is helpful in physics experiment to determine the unknown resistance of wires like Constantan and Nichrome etc. With the help of Nvis 6041, the student can learn the principle of Post Office Box and its working. It can determine the specific resistance of different types of wire.
Measurement of an Unknown Resistance Using a Post Office Box - Free download as Word Doc (.doc / .docx), PDF File (.pdf), Text File (.txt) or read online for free. This document describes using a Post Office Box circuit to measure an unknown resistance. The Post Office Box acts as a Wheatstone bridge with three known resistances (P, Q, R) and one unknown (S).
To verify laws of resistances in series, a post office box is used to measure unknown resistance. It works on the principle of Wheatstone bridge. P, Q, R and S be the four resistances of the Wheatstone bridge as shown in figure 1. Here, P, Q and R are the known resistances and S is the unknown resistance. A galvanometer is connected between B ...
About a Post office box and its experiment, how determine the value of resistance of a wire by using Post office box.A Post Office Box is abbreviated as PO B...
Specific Resistance of the Material of a Wire using Meter Bridge and Post Office Box. By Jitender Singh on Nov 2, 2019. The resistance of a wire of length l and diameter D is given by R = ρl A = 4ρl πD2, R = ρ l A = 4 ρ l π D 2, where ρ ρ is resistivity (or specific resistance) of the material of the wire. The unit of resistance R is ...
A Post Office Box can also be used to measure an unknown resistance. It is a Wheatstone Bridge with three arms P, Q and R; while the fourth arm(s) is the unknown resistance. P and Q are known as the ratio arms while R is known as the rheostat arm. Post Office Box, Physics named it so because it has a shape of box and was designed to find ...
1)During the experiment, due to Joule heating, temperature of the wire increases. Hence, the resistance doesnot remain constant, it changes. So, to get more accurate readings, the circuit should be broken and the wire should be given time to cool down. 2)For, minimum error, the ratio l 100−l → 1. Hence, for this condition, l → 50.
1. The document describes an experiment to determine the resistance of a Weston type galvanometer using Kelvin's method with a Post Office box. 2. A Post Office box is a compact form of Wheatstone's bridge used to measure resistance. The experiment connects the galvanometer as an unknown resistance in one arm and varies resistances in the other arms to obtain balance points. 3. By recording ...
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Post office boxes were common pieces of scientific apparatus in the UK O-Level and A-Level schools public examination physics syllabus in the 1960s. Construction. A typical post office box is in a wooden box with a hinged lid and a metal or bakelite panel showing circuit connections. Coils of wire are wound non-inductively, mounted in the body ...
Hint: A post office box is a Wheatstone bridge composed of three known arms' resistances and one unknown resistance, whose value we calculate using the working principle and expression for a post office box. Complete step-by-step solution: A Post Office Box is used to measure an unknown resistance. It is basically a Wheatstone bridge with three arms known as P, Q, and R; while the fourth arm ...
Experiment 2: Band Gap Determination using Post Office Box Aim : To find the band gap of the material of the given thermistor using a post office box. Apparatus Required : Thermistor, thermometer, post office box, power supply, galvanometer, insulating coil and glass beakers.
During an experiment with a metre bridge, the galvanometer shows a null point when the jockey is pressed at 40. 0 cm using a standard resistance of 90 Ω,as shown in the figure. The least count of the scale used in the metre bridge is 1 mm. The unknown resistance is
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Presented By : Prerna RautB.Sc. V SemesterGovt Autonomous PG College Chhindwara MPIf you like our video, please Like, Share and Subscribe.& Press the bell ic...
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Istra is famous for its New Jerusalem Monastery which was established to serve as a Russian version of the Holy Land. Today the monastery has been completely restored following the damage it suffered at the hands of the Nazis and Bolsheviks, and remains the main reason for visiting Istra. The city can easily be visited as a day trip from Moscow.