How Does a Metal Sleeve Affect a Solenoid's Inductance?

[Antimatter90]

Can anyone explain the concept behind changing of inductance of a solenoid (when AC current is passing through solenoid) on moving a metal sleeve (hollow metallic pipe) over the solenoid enclosing the solenoid.

 

[otto9K9otto]

The conductive metal sleeve is causing energy loss when the magnetic field is changing. It acts, basically, as a shorted turn on the solenoid. This reduces the magnetic coupling of the other solenoid turns, thus reducing the effective inductance.