The resistance of any electrical device is defined by the expression, R=V/I, where V (volts) is the potential difference across the device and I (amps) is the electrical current flowing through it.

When a current of one amp flows through a resistor after a potential of one volt is placed across it we say that the resistor has a unit resistance of one ohm. If the resistor is a metallic conductor then, as long as it's temperature is constant it's resistance remains constant. In other words, the ratio of potential difference (V) and current flow (I) remains constant. This is Ohm's Law. This means that for any given resistor doubling the potential potential across its ends will double the current flowing through it, tripling the potential will triple the current, etc.

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