resistance and temperature effect

A sample of conductor is said to obey Ohms law if its resistance R remain constant that is , the graph of its V verses I is exactly a straight line.

Conductor which strictly obeys the ohms law is called ohmic.Howe ever,there are devices,which do not obeys the ohms law they are non-ohmic devices.

Lets us apply certain potential difference across terminals of a filament lamp and measure the resulting current passing through it.if we repeat the measurements for different values of potential difference and draw a graph of voltage V verses current I it will be seen that a graph is not straight line. it means that graph is non ohmic device.The deviation of I-V graph from straight line is due to increase in the resistance of the filament with temperature.A current is passing through filament is increase from zero,the graph is straight line in initial stage because due to increase in resistance of the filament with the temperature due to small current is not appreciable. As the current is further increased,the resistance of the filament continues to increase due to rise in temperature.