Another alternative is to use a ferrite core, which is a nonconductor.
From the en.wikipedia.org
He is a resolute nonconductor of electricity.
From the theatlantic.com
The point about an AC-cable sheathed in a nonconductor and a metal reminds me of the story of the first transatlantic telegraph cable, which had a similar construction.
From the economist.com
The pith is a nonconductor, so the electrons in the ball are bound to atoms of the pith and are not free to leave the atoms and move about in the ball, but they can move a little within the atoms.
From the en.wikipedia.org
More examples
Insulator: a material such as glass or porcelain with negligible electrical or thermal conductivity
An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. ...