Such vibrating electric bells will work with both direct and alternating currents, but not nearly so well with the latter. As, however, it is very often found desirable to use alternating currents, a special polarized bell has been devised for use with them, in which the armature, the amplitude of whose movement is regulated by screws, vibrates in a vertical plane between two poles of an electro-magnet, bent round at right angles.

No make-and-break mechanism is necessary as in ordinary bells, because the alternating current is continuously reversing the sign of the poles of the electro-magnet, causing the armature and bell-hammer to vibrate, by alternate attraction and repulsion. It is usual in these bells to intensify the sound by using two domes, making use of the return stroke of the hammer.

The current is produced by a small magneto dynamo worked by hand. This method of dispensing with primary batteries is due to Siemens, and has the advantage that a high E.M.F. (Electric Motive Force) can be generated at no expense, the power being supplied by the operator himself.