A metre bridge is a device commonly used in physics experiments to measure the resistance of a circuit element, such as a resistor or a wire. It consists of a long, thin wire stretched over a wooden board, with two conducting terminals, or “jockey” contacts, that can be moved along the wire to change the length of the wire between them. By connecting the circuit element in series with the wire and a power supply and using a galvanometer to measure the current passing through the circuit, one can determine the resistance of the circuit element using the principles of Ohm’s law and the Wheatstone bridge.
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Metre bridge: History and development
The metre bridge was first developed by English physicist Sir Charles Wheatstone in 1843 as a modification of his earlier invention, the Wheatstone bridge. The Wheatstone bridge was a circuit that used four resistors in a diamond shape to measure an unknown resistance by balancing the voltage drop across two opposite sides of the diamond with a known voltage source.
Wheatstone’s modification was to replace one of the resistors in the diamond with a variable resistor, or rheostat, and to replace the voltage source with a galvanometer, which measures the current flowing through the circuit. By adjusting the rheostat until the galvanometer reads zero, one can find the value of the unknown resistance using the ratio of the two fixed resistors in the diamond.
The metre bridge is a simplification of the Wheatstone bridge, in which the diamond shape is replaced by a straight wire and the rheostat is replaced by the wire itself. By measuring the voltage drop across a known length of the wire using a voltmeter, and then moving the jockey contacts to another length of the wire and measuring the voltage drop again, one can find the resistance of the wire between the two jockey contacts using the same ratio method as in the Wheatstone bridge.
Metre bridge: Applications
The metre bridge is a versatile and widely used instrument in physics labs for measuring the resistance of a variety of circuit elements, from simple resistors to more complex circuits such as parallel and series combinations.
It is also used to measure the resistivity of a material, which is its inherent resistance per unit length and cross-sectional area, by varying the length and thickness of the wire and measuring the resistance. Resistivity measurements are important in materials science and engineering for characterising the electrical properties of metals, semiconductors, and insulators.
The metre bridge can also be used to measure the temperature coefficient of resistance of a material, which is the rate at which its resistance changes with temperature. By heating or cooling the material and measuring its resistance at different temperatures, one can determine its temperature coefficient and use it to calculate the temperature at which the material will have a certain resistance.
FAQs
How accurate is the metre bridge in measuring resistance?
The accuracy of the metre bridge depends on the quality of the wire, the jockey contacts, and the galvanometer used, as well as on the skill and experience of the user. With good equipment and technique, the metre bridge can achieve an accuracy of about 1% or better.
Can the metre bridge be used to measure capacitance or inductance?
No, the metre bridge is designed specifically for measuring resistance and cannot be used to measure capacitance or inductance directly.
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