CONSTRUCTING A STANDARD RESISTANCE

CONSTRUCTING A STANDARD RESISTANCE

AIM:

To construct a standard resistance of a given value

APPARATUS REQUIRED:

2 Cells or a battery eliminator,a plug key,resistance wire of Constantine and of appropriate diameter,an ammeter, a voltmeter,a wire cutter etc.

THEORY:

The resistance ‘R’ of a wire is directly proportional to its length ‘l’ and inversely proportional to its area of cross section ‘a’, mathematically,

R α  l

R α l/a

R=δl/a

Where δ is the constant of proportionality and is called resistance or the resistivity of the material of the wire and its SI unit is ohm-metre

From the equation it is clear that for making a standard resistance of known value, one must know the 3 quantities δ,l and a. For making the standard resistance, the wire choosen should neither be too long nor too short. About 40cm is a convenient length for making a standard resistance.

Thus by using the known values, the diameter of the wire required is computed. The computed diameter is related to standard wire guage number

PROCEDURE:

[a] EXPERIMENTAL DETERMINATION OF A RESISTANCE OF A KNOWN LENGTH OF THE WIRE SELECTED

1. take a length little more than 40cm of the wire selected for making the resistance

2. determine the resistance by ohm’s law using the circuit diagram

3. measure the exact length of the experimental wire between the terminals. Do not take the into account the length of the wire pressed under the terminals.

[b]COMPUTING THE LENGTH CORRESPONDING TO 5 OHM RESISTANCE

4. Using the exact length of the wire and the value of its resistance determined above  , calculate the length required for making a resistance of 5 ohms.

[c] NON-INDUCTIVE WINDING OF WIRE OVER A BOBBIN

5. take the above wire and double it to avoid self induction effects

6. take a bobbin of non conducting material. Wind the wire on the bobbin leaving its ends free

7. fix the bobbin on a wooden board of a suitable size and connect the free ends of the wire to the terminals T1 and T2 provided on the wooden board.

   Now your standard resistance of 5 ohm value is ready

[d] CHECKING THE ACCURACY OF THE STANDARD RESISTANCE

8. Use the circuit diagram given by replacing the unknown resistance R by this standard resistance determine accurately its value.