I've been reading up on tesla's wireless power transmission lately, and I've been wondering how magnetic flux through the center of a toroid induces a voltage in its coils. From what I understand, the time derivative of magnetic flux through a loop is proportional to the induced voltage, however; in this case, the coils will be parallel to a field passing through the center of the toroid - meaning no flux through any of the coils. But what I've read about these toroid inductors suggests that there is an induced voltage. Could someone explain this? Also, how does the induced voltage compare to that of a solenoid with an equal number of turns? i.e. is the toroid more efficient / effective? thanks!