Meter Bridge for IIT JEE

Difference Between Potentiometer and Meter Bridge

There are several things that happen in our day to day life that amazes us. There are numerous experiments that show creative ways to use things and build newer things from them. Meter Bridge is one of those few interesting experiments in science that fills us with curiosity and amazement. In this article we will be learning more about the concept of the Meter Bridge in details. 

What is basically a Meter Bridge?

A meter bridge is observed to be a simple form of potentiometer that is used in science laboratories in schools and colleges. This device is very useful in the determination of the principle of measurement of resistance by potentiometric means.
Here in this device, a wire of an unknown resistance is used and is connected to a galvanometer by the use of a slider. When the reading of galvanometer is zero, then the ratio between the wire length to the right and left sides of the slider becomes equivalent to the ratio of an identified and unidentified resistor in a parallel form of a circuit. 
A Meter Bridge is popularly called as a slide wire bridge. The Wheatstone meter bridge was invented in the year 1833 by a famous scientist Samuel Hunter Christie which was later enhanced and popularized by scientist Charles Wheatstone in the year 1843.

A slide wire bridge is basically a type of a resistance measuring device that is used in the form of a Wheatstone bridge. It consists of a spiral or a straight spiral shaped wire that is divided into two parts by using a sliding contact that forms the complete or identified regions of head-to-head arms of the meter bridge. 
Meter Bridge is also termed as a Wheatstone's meter bridge. It is a device that is useful in identifying the unidentified resistance of any wire in a Wheatstone bridge and to perform a comparison between two unknown resistors.

The apparatus of a meter bridge comprises of connecting wires, a one-way key, a scale, an unknown Resistor, a Resistance Box or a known resistor, a jockey and a Galvanometer. This entire experiment is carried out to measure the specific resistance of the material of the wire. The length of the wire that is used in performing this experiment is of one meter.

Let us see how to do a connection of this circuit:

We use a resistance box as R and an unidentified resistance as “S” to connect across the 2 gaps of metallic copper strips. Now connect one of the ends of the galvanometer device to the mid lead of a metallic strip. 

This strip is placed in between the two copper strips with the shape of L. Now you need to connect the other end of a galvanometer to a jockey. This jockey is later used to easily slide on the wire of the bridge. A jockey is basically a metallic rod that has one of its ends as the edge of a knife.

To calculate the unknown resistance of a wire, you will be required to connect all the above devices and the wires in the correct way. This will form a circuit. In the starting, the meter reading of the galvanometer will be zero. In other words, it does not show any deflection. 

At this time, you need to measure the length of the unknown resistor. Let us name this length as L1. Once you do this, you will need to adjust the resistance value in the resistor. The value of the variable resistor can change with the application of the voltage to the circuit. Now slide the jockey throughout the wire. 

Now measure the wire length, when the voltage is applied through the circuit. The galvanometer shows null deflection. Let us name this length as L2. Now once you get both these lengths, you can use the Wheatstone bridge principle to find the unidentified resistance of a Meter Bridge. 

This unknown resistance wire has uniform cross-sectional area such that L1 + L2 = 100. This wire is properly fixed on a wooden based block. The material with which this wire is made is manganin or constantan.

With the technological advancement, nowadays it has become possible to perform different types of measurements by using sophisticated machines and tools. However, still today, the wheat bridge is an authentic means to calculate the resistance of an electric circuit (to even the nearest milli-ohms with complete accuracy).

Assuming that L1 = L = 100 – L
Now, unidentified resistance “X” of a given wire: X = R * L2 / L1 = R (100-L) / L
The specific resistance (R) of the wire material is obtained by the above relation as: (3.14) * (r *2 X) / L
Here: r is the radius of the wire, and l is the length of the wire.

To find the correctness of the output, you can interchange or swap the placement of the unknown as well as known resistance in the bridge. If your bridge is symmetrical and working properly and rightly, then you will be getting the same value of resistance in this case too. If you get different value in both the cases, then the bridge is not symmetrical. 

Why is manganin or constanton chosen as the material of the wire?

To perform a meter bridge experiment, the wire is made up of manganin or constantan because of its high resistivity or specific resistance. The material constantan was started to be used in performing experiments by Edward Weston in the year 1892. Foil made up of Manganin is mostly used in the formation of resistors, especially ammeter shunts. This is because of the virtually 0 temperature co-efficient of the value of the resistance and its longer-term stability.

Why does it make sense to use copper wires in performing an experiment?

Thick strips of copper are used in this bridge as copper is considered to be a favourable conductor of electricity. The reason for using broad metal strips in the Meter Bridge is to lessen the amount error measured in the unknown resistance’s value.

Why this bridge experiment called as a meter bridge?

The reason why this bridge is called as Meter Bridge is because of the fact that it works on the Wheatstone bridge’s principle. So, people also popularly call it as Wheatstone's meter bridge. Another thing is that the wire length used in this circuit is of 1 meter, so it has got its name as a meter bridge.

What kinds of end errors are reported in a meter bridge?

One of the commonest forms of errors that are reported in the meter bridge experiment is the shifting of 0 on the galvanometer scale at varying points and the stray resistance that results into the formation of end error in the wire of the meter bridge wire.

The basic reason behind this error is due to the non-uniformity of the wire. End corrections are projected by inclusion of known resistances at the two ends and then locating the null point.

What is measured as a null point in a Meter bridge?

There is a null point observed in case of a meter bridge experiment. This “null point” is located at a distance of forty centimeters from the point A. On application of a resistance of magnitude 12 ohm to the circuit in parallel to the variable resistance, a null point is formed and measured at 50.0 cm from A. 

What do you mean by balancing length in a meter bridge?

Balancing length is an important concept in a meter bridge experiment. The value of balancing length “l” is seen to remain constant when the radius of the meter bridge wire gets doubled and keeping the cross-section area of the wire uniform. This happens due to the relation R/X = l/(100−l), being independent of the cross-section area of the wire. Here, X is an unknown resistance.

What are the practical applications of a Wheatstone meter bridge?

Wheatstone bridge is one of the most popular bridges in the history of science. There are several different applications of this bridge in invention of various new scientific principles. The use of this bridge includes thermistor, strain gauge, light detector, potentiometer, etc. Wheatstone bridge also has its significance in an operational amplifier due to its ability to measure and magnify the tiny changes that occurs in resistors.

In what ways does a potentiometer differs from a meter bridge?

Both potentiometer and a meter bridge are widely used devices in the calculation of several scientific theories and inventions. They have a huge significance in science. A Meter Bridge is a kind of an electric device that assists in the measurement of an unidentified resistance. 

On the other hand, a potentiometer is also an electric device that is used for the measurement of the internal resistance and the EMF or electro motive force of cells. It helps in the comparison of emf of two cells along with a potential difference that forms between 2 points in a given electric circuit. However, it has been found that the accuracy is greater in a potentiometer because of the use of longer length of resistance wire.