The resistance of a metal, that is the ratio of potential difference in volts to the current, is constant providing the temperature remains constant. This is another way of stating Ohm's Law. Unfortunately if the temperature changes or if the part you are using is not a metal then this simple connection might not be true.
The notes below explain Ohm's Law and what is likely to happen if the temperature changes using the examples of a filament lamp and a thermistor. There are also notes on the characteristics of a diode, that is explaining the changing current as the potential difference is altered.