a permanent magnet, so that this is alternately attracted and repelled by each half-phase and different polarity of the supply. In practice, the armature is arranged symmetrically with two poles of opposite polarity facing each end of the coil, so that each may be attracted in turn. No contact breaker is required, so such bells are reliable for long service. In some countries, notably the UK, the clapper struck two different sized bells in turn giving a very distinctive ring.

Fire alarms

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