Colour code, ohmic and non ohmic resistance

# Colour code:

The value of resistance lies between fews ohms to many thousands ohms of different colours colours are marked on the carbon resistance. Colours of two bands A and B represent significant value of carbon resistors in ohms. Colour of third band C represents the Indices of 10 and fourth band D gives the error in calculation of resistance in percentage.

# Ohmic Resistance:
The conductors which obey ohm's law are called Ohmic conductor is called Ohmic Resistance.

Exa: Resistance offered by metals.
         Al , Au, Ag , Cu etc

For Ohmic Resistance the graph drawn between V and I is always a straight line

2) Non Ohmic Resistance:
The conductors which doesn't obey ohm's law are called Non Ohmic conductors and the resistance offered by them are called Non Ohmic resistance.

Exa: Resistance offered by semiconductor.
         (Si , Ge) , vacuum tube etc.

The graph between V and I for non Ohmic
resistance is not a straight line.

Note:  The resistivity of alloys is high and temperature coefficient of resistivity is very small. Thus alloys are used for making standard resistance coil and resistance boxes.

Some alloys used as standard resistance are follow--

1) Manganin (Alloy of Mn, Cu ,Ni)
2) Constantan (Alloy of Cu, and Ni)
3) Nichrome (Alloy of Ni, Fe and Cr)