 When analyzing the response of materials to alternating electric fields (dielectric spectroscopy), in applications such as electrical impedance tomography, it is convenient to replace resistivity with a complex quantity called impedivity (in analogy to electrical impedance).Impedivity is the sum of a real component, the resistivity, and an imaginary component, the reactivity (in analogy to reactance).The magnitude of impedivity is the square root of sum of squares of magnitudes of resistivity and reactivity.