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MEASUREMENT OF RESISTANCES

INTRODUCTION

CLASSIFICATION OF RESISTANCES

MEASUREMENT OF MEDIUM RESISTANCES

Expression for Galvanometer current

Errors in WS bridge measurements

Limitations of WS bridge

MEASUREMENT OF LOW RESISTANCES

INTRODUCTION

A resistance is a parameter which opposes the flow of current in a closed electrical
network. It is expressed in ohms, milliohms, kilo-ohms, etc. as per the ohmic principle, it is
given by;
R = V/I; with temperature remaining constant. (1)
If temperature is not a constant entity, then the resistance, R2 at t2
0C is given by;
R2 = R1 [1+  (t2-t1)] (2)
Where t= t1-t2, the rise in temperature from t1
0C to t2
0C,  is the temperature coefficient of
resistance and R1 is the temperature at t1
0C.
Resistance can also be expressed in terms of its physical dimensions as under:
R = l/A (3)
Where, l is the length, A is the cross sectional area and  is the specific resistance or
resistivity of the material of resistor under measurement.
To realize a good resistor, its material should have the following properties:
• Permanency of its value with time.
• Low temperature coefficient of resistance
• High resistivity so that the size is smaller
• Resistance to oxidation, corrosion, moisture effects, etc.
• Low thermo electric emf against copper, etc.
No single material can be expected to satisfy all the above requirements together. Hence, in
practice, a material is chosen for a given resistor based on its suitability for a particular
application.