## Aim/Aim of Experiment

To verify the Laws of Combination (Parallel) of Resistances using a Metre bridge.

## Apparatus/Material Required

- A Meter bridge
- A Leclanche cell (Battery Eliminator)
- A Galvanometer
- A Resistance box
- A Jockey
- Two Resistance coils/wires
- A Set square
- Sand Paper
- Connecting wires.

## Theory

(i) The resistance (r) of a resistance wire or coil is given by r = (100-l)/L×R,

Where R is the resistance from the resistance box in the left gap and L is the length of the Meter bridge wire from zero end up to balance point.

(ii) When r_{1} and r_{2} are connected in parallel then their combined resistance is given by

R_{P} = r_{1}r_{2}/r_{1}+r_{2}.

## Circuit Diagram

## Procedure

- Mark the two resistance coils as r
_{1}and r_{2}. - To find r
_{1}and r_{2}process same way as in experiment 2 (if r_{1}and r_{2}unknown). - Connect the two coils r
_{1}and r_{2}in parallel as shown in figure in the right gap of Meter bridge and find the resistance of this combination, Take at least three sets of observations. - Record your observations.

## Observations

**Value of r _{1}:**

Resistance of R.B (ohm) | Balance L (cm) | (100-l) (cm) | [(100-l)*R]/L | Mean r_{1} |
---|---|---|---|---|

0.5 | 59 | 41 | 0.347 | |

1 | 37 | 63 | 0.587 | 0.6393 |

2 | 33 | 67 | 0.985 |

**Value of r _{2}:**

Resistance of R.B (ohm) | Balance L (cm) | (100-l) (cm) | [(100-l)*R]/L | Mean r_{2} |
---|---|---|---|---|

0.5 | 83.2 | 16.8 | 0.1009 | |

1 | 80.8 | 19.2 | 4.208 | 1.729 |

2 | 69.4 | 30.6 | 0.881 |

**Value of r _{3}:**

Resistance of R.B (ohm) | Balance L (cm) | (100-l) (cm) | [(100-l)*R]/L | Mean r_{p} |
---|---|---|---|---|

0.1 | 27 | 73 | 0.27 | |

0.2 | 32 | 68 | 0.42 | 0.36 |

0.5 | 56 | 44 | 0.39 |

## Calculations

Calculations for verification of laws r_{1} and r_{2} in parallel:

- The Experimental value of R
_{p}= 0.36 ohm. - The Theoretical value of R
_{p}= (r_{1}r_{2})/r_{1}+r_{2}= (0.693×1.729)/(0.693+1.729) = 0.4667 ohm. - Experimental Error (Difference if any) = [(0.36-0.4667)/(0.36×100)] = 29.6%.

## Result

Within limits of Experimental Error, Experimental and Theoretical values of Rp are same, Hence, low of resistances in parallel are verified.

## Precautions

- The Connection should be neat, clean and tight.
- All the plugs in the resistance box should be tight.
- The wire should not make a loop.
- The key should be insert only while taking observations.
- Check source of error.

## Sources of Error

- Resistance box, Instrument screw and other plugs may be loose.
- Unavailable thickness connecting wires.
- Wire may be make loop.

## Viva Voice Questions with Answers

**1.** What is resistance?

**Answer:** The ratio of potential difference V across the ends of a conductor to the current I flowing through it. It is represented by R and is given by R=V/I.

**2.** List the factors affecting the resistance?

**Answer:** The factors affecting the resistance are,

- Length
- Material
- Area of cross-section
- The temperature of the conductors

**3.** What is the mathematical form of Ohm’s law?

**Answer:** V=IR

**4.** What is ohmic resistance?

**Answer:** The resistance which obeys ohms law is known as Ohmic resistance.

**5.** Give examples of non-ohmic resistance?

**Answer:** Transistors, vacuum tube diodes and semiconductor diodes.

**6.** Why do we use thick connecting wires?

**Answer:** Thick connecting wires offer negligible resistance compared to given alloy wire whose resistance is to be determined.

**7.** What is the material of the connecting wires used in the experiment?

**Answer:** Copper.

**8.** How does resistance change in parallel combination?

**Answer:** Resistance decreases in parallel combination.

**9.** Explain decrease of resistance in parallel combination.

**Answer:** In parallel combination, the effective area of cross-section increases. As R∝ 1/A resistance decreases in parallel combination.

**10.** What is Wheatstone bridge?

**Answer:** It is the arrangement of four resistance in quadrilateral form to determine one unknown resistance in term of other three resistances.

**11.** What is a metre bridge?

**Answer:** It is the practical form of Wheatstone bridge to determine the unknown resistance and resistivity of a given alloy wire.

**12.** Why is a metre bridge so called?

**Answer:** Since the bridge uses one metre long wire, it is called a metre bridge.

**13.** Why the bridge method for resistance measurement is better than Ohm’s Law?

**Answer:** It is so because the bridge method is a null method (at null point, there is no current flowing in galvanometer) and more sensitive.

**14.** Why the metre bridge is suitable for measuring moderate resistances?

**Answer:** Because, Wheatstone bridge is suitable for moderate values of resistances. Therefore, Meter Bridge is more sensitive for moderate values.

**15.** Why is Wheatstone bridge (or metre bridge) method considered unsuitable for the measurement of very low resistance and very high resistance.

**Answer:** (i) For measuring low resistance – All resistances and resistance of galvanometer should be f low. The end resistance and connecting wires become comparable to the resistance being measured and introduce error in the result.

(ii) For measuring high resistance – The resistance forming the bridge should be high and the current in the galvanometer reduces and it became insensitive.

**Class 12 Physics Practicals:**

- To Determine Resistance Per cm of A Given Wire by Plotting A Graph for Potential Difference Versus Current
- To Find The Resistance of A Given Wire using The Metre Bridge and Hence Determine The Resistivity (Spacific Resistance) of It’s Material
- To Verify The Laws of Combination (Parallel) of Resistances using A Metre Bridge
- To Verify The Laws of Combination (Series) of Resistances Using A Metre Bridge
- To Compare The EMF of Two Given Primary Cells Using Potentiometer
- To Determine The Internal Resistance of A Given Primary Cell Using Potentiometer

What is r1 and r2?

r1 is 1st resistor and r2 is the 2nd resistor.