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Wheatstone bridges are electrical circuits used to measure unknown electrical resistance by balancing their two legs, where one of the legs includes a novel component. Sir Charles Wheatstone improved and simplified this instrument in 1843 after it was constructed by Samuel Hunter Christie in 1833. In today’s world, digital multimeters provide the easiest way to measure resistance. Wheatstone bridges can still be used for measuring resistances whose values are within the range of milli-Ohms. A metre bridge is an apparatus used to determine a coil’s unknown resistance. Find the resistance of the given wire by metre bridge of a uniform cross-section of about 1m length replaces one of the lateral resistances in the metre bridge.
About Metre Bridge
The Wheatstone bridge idea is based on the metre bridge idea called a slide wire bridge. While theoretical knowledge is the basis for our understanding of Physics, practical experience is the basis. For the same reason, several experiments will be performed in the school laboratories. A galvanometer, resistance box, and a switch are often part of practical labs performed. What are these devices? These are metres! A metre bridge has a wire of 1 metre, thus its name. Using a resistance metre, wires, coils, and other materials can be tested for resistance.
Metre Bridge Principle
Wheatstone bridges work on the same principle as metre bridges. Wheatstone bridges work over the null deflection of principle, i.e., no current flows middle arm when the resistance ratios in the two arms are equal.
Metre Bridge Construction
1. Wires on the metre bridge have a uniform cross-sectional area and are tightly coiled.
2. This wire is then carefully clamped in two strips of metal bent towards right angles.
3. The found resistance of the given wire by metre bridge is then connected to the metal stripes. There are two gaps, one containing a box of resistance R and the other containing a wire of small resistor S.
4. Through the cell, the clamped endpoints of the wire are attached to a key.
5. A galvanometer attached to the right metallic is in the middle of the two gaps.
6. An electrical connection is made by connecting a jockey. In this example, the jockey is a metal rod with a knife-like edge as the other end of the galvanometer on one end that potentiometer wire slides over it. This can be seen on the metre bridge diagram.
Working Process
1. Start by placing the jockey at the endpoints of the wire; make sure the deflection of the galvanometer is the same deflection at both ends.
2. Jockey slides slowly through the wire from side to side, paying close attention to where the galvanometer deflection stops.
3. You can try changing the different resistance to see if you can reach a point.
4. Look after the point over the wire where the galvanometer shows zero deflection as you slide the jockey over the wire.
5. Using the metre scale attached to the wire, measure the length of the null point. It is the length that balances the metre bridge.
6. A and B are separated by “l1“.
Distance l2 between points B and C equals 100 minus l1 in points B and C.
Formula To Determine Unknown Resistance
Choose a suitable type of resistance from the resistance box, ‘R.’ For metre bridge formula
Contact point A in the jockey of wire; look for a deflection in the galvanometer on one side; then contact the jockey at point C of wire; the deflection in the galvanometer should be on the other side.
Determine the null point that has no deflection in the galvanometer. A and B are equal to (100 – l).
To find the unknown resistance use the formula:
S = (100 – l1)R/l1
You can calculate the resistivity with formula:
Ρ = πd2S/4L
Here, d is actually the wire’s diameter, and S is the resistance that is unknown.
Follow the same procedure for other values of R. Find the resistance of the given wire by metre bridge at least five measurements.
It is called the point of balance when the galvanometer shows no deflection.
Take five readings of the wire given using an ordinary scale and a screw gauge to measure the wire’s radius. (Use a minimum of five readings).
Multiply the sum of the resistances from the five readings of an unknown resistance by five to calculate the Mean Resistance. This is how to find the resistance of the given wire by a metre bridge.
Conclusion
Bridge wire is spelt out on a scale along its length. The 100cm mark on the scale will not collide with the endpoint of the wire of the bridge if the zero of the scale is not aligned with the wire’s starting point. Consequently, the length of the balancing measurement will be inaccurate. We can measure unknown resistance values with the help of finding the resistance of the given wire by metre bridge. The wire is uniformly cross-sectioned and lengthened by a metre. There are several kinds of wire available, including nichrome and manganin. The principle of metre bridges is identical to that of Wheatstone bridges. Null deflection is the principle behind Wheatstone bridges. Hence the unknown resistance, S=(100–l1)Rl1.
