In watchmaking, the term cage indicates two technically distinct structures: the tourbillon cage and the antimagnetic Faraday cage. Confusing them is a common mistake, even among enthusiasts.
The tourbillon cage is the rotating frame that houses the balance wheel and hairspring. Breguet patented the principle in 1801: by putting the regulator in continuous rotation, generally one revolution per minute, it compensates for the running errors caused by gravity in different positions. The traditional cage is supported by a lower and upper bridge. In the flying tourbillon, also called floating tourbillon, the upper bridge is eliminated and the cage is cantilevered from only one side: the mechanism's visibility improves, the weight reduces, and the distribution of mechanical stresses changes in non-trivial ways. I disassembled numerous Piaget flying tourbillons in the Seventies and Eighties: the machining precision required to keep a cantilevered cage in alignment is an order of magnitude higher than the classical model. There are then variants that push the concept further: the triple-axis tourbillon rotates around three axes simultaneously at different speeds; the armillary tourbillon adopts a spherical structure reminiscent of celestial circles; in the Exo Tourbillon the balance wheel is positioned outside the cage, allowing for a larger diameter balance and a lighter cage; the cylindrical tourbillon adopts cylindrical geometry instead of circular. De Bethune brought the cage down to 0.18 grams with 63 titanium components. The Bulgari Octo Finissimo Ultra guides the cage laterally with nine ball bearings, eliminating both bridges and distributing everything on a single horizontal plane to keep the thickness at 1.18 mm.
The Faraday cage, on the other hand, is an antimagnetic shielding: a soft iron structure, sometimes integrated into the inner case, sometimes built as a plate under the dial, that absorbs external magnetic fields without becoming permanently magnetized. You find it in the IWC Ingenieur from the Seventies, in the Rolex Milgauss, in the Laventure Marine Type 3. Soft iron works because it has high magnetic permeability: the field is channeled into the metal and diverted away from the movement. It's not about blocking magnetism, but offering it a preferred path that bypasses the movement.
The two cages have nothing in common except the name: one solves a gravitational problem, the other an electromagnetic one.