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.
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