Homoeoid and focaloid

[1][2] When the thickness of the shell becomes negligible, it is called a thin homoeoid.

The name homoeoid was coined by Lord Kelvin and Peter Tait.

[3] Closely related is the focaloid, a shell between concentric, confocal ellipses or ellipsoids.

The gravitational or electromagnetic potential of a homoeoid homogeneously filled with matter or charge is constant inside the shell, so there is no force on a test particle inside of it.

[5] Meanwhile, two uniform, concentric focaloids with the same mass or charge exert the same potential on a test particle outside of both.