What causes sonometer wire to vibrate in A.C Sonometer experiments?

[Amith2006]

Sir,
In an A.C Sonometer experiment, what is the principle on which the sonometer wire vibrates when an A.C voltage and a magnetic field is applied between any 2 points of the wire?

 

[pallidin]

The applied AC voltage generates a constantly varying magnetic field, which engages and reacts with the static external magnetic field, causing a force reaction in BOTH the AC wire and the external magnet. The wire, often being very thin and of a mass much lower than the external magnet, exhibits a noticable cyclic displacement.
Careful measurements would show that the external magnet also vibrates from this scenario.