For a fixed metal conductor, the temperature and other condition remaining constant the current through it is proportional to the potential difference between conductor ends.
V/I = Constant
V = volt
I = Current
R = Resister of the conductor between the two pints considered
we can apply V=IR Linear resistors only.
Current is directly proportional to the applied voltage. Their current versus applied voltage is straight and linear. In other words, their resistance remains constant.
Resistors are those whose current does not change linearly with changes in applied voltage.