Question

Why is the Wheatstone bridge better than the other methods of measuring resistances?
A. It does not involve Ohm’s law
B. It is based on Kirchhoff’s law
C. It has four resistor arms
D. It is null method

Answer 1

Hint:Recall the various methods of resistance measurement. Identify the flaws and accuracy of these methods. The Wheatstone bridge uses four resistors out of which three of them are known and one is unknown.

Complete answer:
We know that there are several methods of measuring medium range unknown resistance. These are: 1) Ohm-meter method 2) Voltmeter-ammeter method 3) Substitution method and 4) Wheatstone bridge method.

In Ohm-meter measurement of the resistance, the unknown resistance is simply measured by the Ohm-meter which is powered by the battery. This method does not give accurate results but it is though quick method to measure the resistance. The voltmeter ammeter method is a direct application of Ohm’s law. We measure the current through the resistance using the ammeter and the potential drop across the resistance using the voltmeter.

Then using Ohm’s law, the value of resistance can be determined. This method also does not give the accurate result as the voltmeter measures the potential drop across the resistance as well as ammeter. Substitution method involves substitution of another parallel resistance across the unknown resistance. This method is also not reliable since it depends upon the sensitivity of the ammeter.

Now, in the case of Wheatstone bridge, the four resistances are connected in the four arms of the rectangular circuit with unknown resistance at one of the arms. We know that Wheatstone bridge works on galvanometer null deflection method. By varying the known resistance in one of the arms, the circuit is balanced such that the current through the galvanometer becomes zero. At this point, the bridge balance condition gives the correct value of unknown resistance. Since the Wheatstone bridge is a null point method, it does not involve the use of ammeter or voltmeter.

So, the correct answer is option D.

Note:Galvanometer is a current sensitive device and thus detects a very small amount of current flowing through it. In case of ammeter and voltmeter, they do not give accurate measurement of current and voltage respectively but they are usually used because they give the value of measurement in no time. The arrangement of Wheatstone bridge takes time to construct the circuit.