The resistance of a sample of material, measured in units of ohms (W), is defined as the ratio of voltage (in units of volts) across the sample of material to the current (in units of amperes) through the material. The name ampere is usually abbreviated to amp. When we draw a graph of voltage across the sample (a resistor) plotted against current through the material, the value of resistance is represented by the slope of the graph.
For a metallic material kept at a constant temperature, a straight-line graph indicates that the material is ohmic, obeying Ohm’s law.
Non-ohmic behavior is represented on such a graph by a curved line or a line that does not pass through the zero-voltage, zero-current point that is called the origin. Non-ohmic behavior can be caused by temperature changes (as in light bulbs and thermistors), by voltage generating effects (as in thermocouples and cells) and by conductivity being affected by voltage (as in diodes).
A material that is ohmic will have a constant value of resistance (subject to minor alteration with temperature change) and can be used to make resistors. Resistance values will be either colour coded or have values printed on using the conventions of BS1852: 1970.