The wheatstone bridge is one of the most accurate methods of measuring resistance. It consists of four resistors of resistance R1, R2, R3 and R4 connected as shown below.
![wheatstonebridge](https://i2.wp.com/fixpiecdn.files.wordpress.com/2020/05/wheatstonebridge.jpg?ssl=1)
When no current flows through the galvanometer, the bridge is said to be balanced.
Therefore, p.d across R1 = p.d across R3
And p.d across R2 = p.d across R4
Hence, I1R1=I2R3 ——–(1) and
I1R2=I2R4 ——–(2)
Dividing (1) by (2), we have,
R1R2=R3R4
Where R1 is the unknown resistance, R2 is a fixed resistance of known value, R3 & R4 are variable resistances of known values.
Wheatstone Bridge Slide show. Click slides:
https://www.slideshare.net/slideshow/embed_code/key/rvhQ72HZRCoaZWheatstone bridge from LingalaSowjanya
The Metre Bridge
It consists of a straight uniform resistance wire AB of length 1m stretched along a metre rule. The unknown resistor X is placed at the left side, while the known R is placed at the right side.
![meterbridge](https://i2.wp.com/fixpiecdn.files.wordpress.com/2020/05/meterbridge.jpg?ssl=1)
When the circuit is closed, a point is located along the resistance wire with the jockey when G reads zero.
At this point, X ∞ l1 and R ∞ l2
∴ X = kl1 and R = kl2
Since k is the same,
∴XI1=RI2∴X=RI1I2
Precautions:
- The value of the known resistance Rshould be chosen so that the balance point comes between 30cm and 70cm.
- The battery key should always be depressed before the Gcontact is made on the bridge wire.
- Avery small current should be passed through the galvanometer to avoid damage.
- In other not to destroyed the uniformity of the wire, the jockey should touch the wire
The Potentiometer
A potentiometer consists of uniform resistance wire AB of length 100cm through which a source of emf maintains a steady current.
Since the wire is uniform and current constant,
V∝R∝IV=kL
![potentiometer](https://i2.wp.com/fixpiecdn.files.wordpress.com/2020/05/potentiometer.jpg?ssl=1)
When a potentiometer is used to compare the emfs of two cells, at balance point when G reads zero,
E1E2=I1I2
Merits of the Potentiometer Over the Voltmeter
- The potentiometer is a more accurate in measuring voltage than the voltmeter.
- There is no zero scale error as that associated with pointer instrument like voltmeter.
- It passes no current at the time current is being taken.
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