The phenomenon where, at very high frequencies, the eddy current flow is restricted to an extremely thin outer layer of the conductor is called:

The phenomenon where eddy current flow becomes restricted to a very thin outer layer of a conductor at high frequencies is known as the skin effect. This effect occurs because, at higher frequencies, the electromagnetic fields created by current flow in the conductor penetrate less deeply, causing the majority of the current to flow near the surface. This results in a decreased effective cross-sectional area through which the current can flow, leading to increased resistance and losses.

The skin effect has significant implications in various applications involving AC currents, such as in power transmission and induction heating, where it is essential to understand how current distribution changes with frequency. In contrast, other options mentioned do not accurately describe this phenomenon, making skin effect the correct choice. Understanding the skin effect is crucial for engineers and technicians working with high-frequency AC circuits and components.