Why Is That Thick Cable Hollow? High Frequency Current Will Not Go Inside
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Alternating current crowds into the outer layer of a conductor and abandons the middle, and the higher the frequency the thinner that useful layer becomes.
What happens in the wire
A steady current spreads evenly across the whole cross section of a conductor. An alternating one does not, and instead concentrates near the surface, leaving the centre carrying almost nothing. The depth within which most of the current flows shrinks as the frequency rises, so at mains frequency in copper it is roughly a centimetre, at broadcast radio frequencies a fraction of a millimetre, and at microwave frequencies a few thousandths of a millimetre. Below a certain thickness the interior of a conductor might as well not be there.
Why the current avoids the middle
The cause is the conductor acting on itself:
- •A changing current creates a changing magnetic field
- •That field exists inside the conductor as well as outside
- •A changing magnetic field induces a voltage opposing the change
- •That opposition is strongest at the centre, where the field is greatest
- •So current is pushed outward to where the opposition is least
- •Faster changes mean stronger opposition and a thinner useful layer
What engineers do about it
The practical responses all amount to increasing surface area rather than cross section. Overhead power lines use aluminium strands around a steel core, since the steel carries the mechanical load and the current only uses the outside anyway. High frequency coils use a special wire made of many fine strands individually insulated and woven so that each strand spends equal time at the surface and in the middle. Radio frequency conductors are frequently hollow tubes, and plating a conductor with silver improves performance because only the plating carries current. Very high frequency systems abandon wires entirely for waveguides.
What it costs
The practical penalty is higher resistance, and quantifying it explains why the effect is worth engineering around. A conductor that carries current only in an outer layer behaves as though it were a thinner conductor, so its resistance rises with frequency even though the metal has not changed. That extra resistance turns current into heat, which wastes power in transmission and generates unwanted warmth in equipment. In a resonant circuit it also blunts the sharpness of the response, which limits how selective a radio receiver can be. Above a certain frequency simply using more copper stops helping at all.
Where else the same physics shows up
The effect explains several things that look unrelated. Induction cooking heats the bottom few tenths of a millimetre of a pan rather than the whole base, which is why the response feels immediate. Induction hardening of gears and shafts deliberately heats only the surface so that the outside becomes hard while the core stays tough. Metal detectors and security screening exploit the same frequency-dependent penetration. Electromagnetic shielding works partly for this reason, since a field cannot get far into a conductor. And the depth of penetration is the reason why low frequency radio reaches submarines and higher frequencies do not.
The takeaway
Alternating current induces opposing voltages inside its own conductor, strongest at the centre, so the flow crowds into an outer layer that thins as frequency rises. Engineers respond by increasing surface rather than cross section, using stranded and woven wire, hollow tubes and silver plating. The same physics gives induction cooking its speed and lets low frequency radio reach submarines.