Ohm's Law is not a universal law, and only applies to a limited range of conductors. For Ohm's Law to apply, the ratio of voltage to current must remain constant for variations in voltage; if the ratio varies, then Ohm's Law does not apply. Materials for which Ohm's Law applies are called 'linear' or 'ohmic'; those for which it doesn't apply are called 'non-linear' or 'non-ohmic'. There are far more 'non-linear' materials than there are 'linear'; these include the metal tungsten, electrolytes, gases, and semiconducting devices, such as diodes, etc. Hence,ohm's law is not applicable for electrolytes and vacuum tubes .