Physically, the conductivity equation is obtained as a low-frequency l
imit of time-harmonic Maxwell's equations. In this work we consider th
e relation of corresponding inverse boundary value problems. The behav
iour of the impedance mapping for time-harmonic Maxwell's equations is
analysed when the frequency goes to zero where Maxwell's equations ha
ve an eigenvalue of infinite multiplicity. We show that an appropriate
restriction of the impedance mapping for Maxwell's equations has a lo
w-frequency limit. Also, we give a formula from which the impedance im
aging data (the Dirichlet-to-Neumann mapping for the conductivity equa
tion) can be calculated by using the low-frequency limit of the impeda
nce mapping.