Ohm's law is true for metallic conductors only.
According to this law, if there is no change in the physical state of a conductor (such as temperature)
then the ratio of the potential difference applied at its ends and the current flowing through it is a constant. If a graph be drawn between the applied potential difference V and the current i flowing through the conductor then it will be a straight line. Ohm's law is true for metallic conductors only. Hence, electrolytes, discharge tubes and vacuum tubes do not obey Ohm's law.
