Ohms law states that at a constant temperature, current I through a conductor between two points is directly proportional to the potential difference or voltage V, across the two points. That is,

                                                 V ∝ I

                                     V/ I = constant = R

                                     V = IR

Thus, the ratio V : I is a constant. This constant is called as the resistance (R) of the conductor.

After performing experiment for different readings of V and I and recording the observations, if we plot current on the x-axis of a graph and voltage on the y-axis of the graph, we will get a straight-line. The gradient of the straight-line graph is related to the resistance (R) of the conductor.