A Meter Bridge also called a slide wire bridge is an instrument that works on the principle of a Wheatstone bridge. A Wheatstone meter bridge is used in finding the unknown resistance of a conductor as that of in a Wheatstone bridge.

The wire is made of manganin because they have low temperature coefficient and large resistivity. The wires are soldered to two thick L shaped copper strips with binding terminals. Between these there is a third copper strip. There are two fixed gaps ab and cd.

(1) It is used to measure unknown resistances $(1 \Omega \text { to } 1000 \Omega)$, for comparision of two unknown resistances and specific resistance of material of wire.

(2) For better accuracy R must be adjusted to obtain L between 40 to 60 cm. It has maximum accuracy when $\mathrm{L} \approx 50 \mathrm{cm}$.

(3) It is more sensitive than PO box but suffers from end resistances.

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### SOLVED EXAMPLES

**Ex.** An unknown resistance S is placed on the left gap and known resistance of $60 \Omega$ in right gap of meter bridge. The null point is obtained at 40 cm from left end of bridge. Find the unknown resistance?

**Sol. $S=R \frac{(100-L)}{L}=60 \frac{(100-60)}{60}=40 \Omega$**

**Ex.** In a meterbridge the length of wire is 100 cm. At what point is balance point obtained if two resistances are in ratio 2 : 3?

**Sol. **$\frac{P}{Q}=\frac{L}{100-L}=\frac{2}{3} \quad$ so $\quad 3 \mathrm{L}=200-2 \mathrm{L} \quad$ or $\quad L=\frac{200}{5}=40 \mathrm{cm}$

**Also Read:**

- Types and Effects of Electric Current
- Ohm’s Law and Resistance
- Combination of Resistances
- EMF and Internal Resistances of a Cell
- Cells Connected in Series, parallel and Mixed
- Kirchhoff’s Circuit Law
- Electric Currents in Conductors
- Moving Coil galvanometer
- Ammeter and Voltmeter
- Potentiometer Working Principle