What is constant in ohms law?

Ohm's original law was 'The potential difference across a conductor is proportional to the current flowing through it, provided physical conditions such as temperature remain constant.'

Today Ohm's law is expressed as E = IR or sometimes V = IR,

the units being Volts, Amps and Ohms.

Ohm's Law ('the current flowing along a conductor, at constant temperature, is directly proportional to the potential difference across that conductor') only applies when the resistance of the conductor is constant so, when verifying Ohm's Law, the temperature must be kept constant, in order to keep the resistance constant.

It should be pointed out that the ratio of voltage (U) to current (R) is called resistance (R), and the resistance of a circuit can be found from the equation, R = U/I whether Ohm's Law applies or not -but Ohm's Law itself only applies when the ratio is constant over a range of voltage variation.