**Aim**: To verify laws of combination (parallel) of resistance using meter bridge .

**Apparatus**: a meter bridge , a leclanche cell , a galvanometer , a jockey , a jockey , a resistance box , two resistance wire , connecting wires , set square .

**Theory**

The resistance (r) of a resistance wire or coil is given by r=(100-l)/L *R where R is the resistance from resistance box in the left gap and L is the length of meter bridge wire from O end upto balance point.

When r_{1} and r_{2} are connected in parallel , then their combined resistance

Rp = r_{1}r_{2} /r_{1}+r_{2}

Procedure

- Mark the two resistance wires as r
_{1}and r_{2} - Connect the two coils r
_{1}and r_{2}in parallel as shown in fig. in the right gap of meter bridge and find the resistance of this combination . Take at least three set of observation - Record your observation .

**Result**

Within limit of experimental error , experimental and theoretical values of R_{p} are same .

Hence law of resistance in parallel are verified .

**Precaution**

The key should be inserted only while taking observation .

Connection should be neat and tight .

Source of error

The galvanometer may not show opposite deflection

The key in the resistance box may be close .

**OBSERVATION TABLE**

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 | 0.6393 |

1 | 37 | 63 | 0.587 | |

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 | 1.729 |

1 | 80.8 | 19.2 | 4.208 | |

2 | 69.4 | 30.6 | 0.881 |

Value of r_{p}

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

0.1 | 27 | 73 | 0.27 | 0.36 |

0.2 | 32 | 68 | 0.42 | |

0.5 | 56 | 44 | 0.39 |

**Calculations**

Rp = (R_{1}R_{2})/R_{1}+R_{2} = (0.693×1.729)/(0.693+1.729) = 0.4667 ohm

Experimental Error = [(0.36-0.4667)/(0.36)]*100 = 29.6%

**Viva question**

1. How does resistance change in parallel combination ?

Resistance decrease in parallel combination

2. explain the decrease of resistance in parallel combination ?

Effective area of cross section increases as r x 1/A , resistance decreases .

3. Why should jockey not be pressed too hard on wire while sliding ?

Sliding the jockey with a hard press will scratch the wire and make its thickness non-uniform .Resistance per unit length of wire will be inconsistent as resistance depends on area of cross section .

4. What is the resistance of an open key ? Explain.

Open key has infinite resistance as it makes current zero (R=V/I)

5. Define unit of resistance .

SI unit of resistance is OHM or volt per ampere . 1 Ohm is the resistance of a conduction carrying 1 ampere current when potential difference maintained across its end is 1 volt .

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