Experimentally, George Simon Ohm found that for most metallic resistors over a wide range of conditions, the current through the resistor is directly proportional to the voltage across the resistor.

This means the resistance is constant over a wide range of conditions.

Ohm also found that resistance changed with temperature. For most metallic resistors the resistance changes linearly with temperature. That is, we can write the resistance as

where R(T) is the resistance at temperature T.

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