where α α is called the temperature coefficient of resistivity, T 0 is a fixed reference temperature (usually room temperature), and ρ ρ 0 is the resistivity at temperature T 0 . The parameter α α is an empirical parameter fitted from measurement data, equal to 1/ κ κ . Because the linear approximation is only an approximation, α α is different for different reference temperatures. For this reason it is usual to specify the temperature that α α was measured at with a suffix, such as α α 15 , and the relationship only holds in a range of temperatures around the reference. When the temperature varies over a large temperature range, the linear approximation is inadequate and a more detailed analysis and understanding should be used.