Alarum

• (n.) See Alarm.
Found on http://thinkexist.com/dictionary/meaning/alarum/

Alarum

A┬Ělar'um noun [ Middle English alarom , the same word as alarm , noun ] See Alarm . [ Now Poetic] » The variant form alarum is now commonly restricted to an alarm signal or the mechanism to sound an alarm (as in an alarm clock.)
Found on http://www.encyclo.co.uk/webster/A/50

Alarum

Horologists seem to prefer this Shakespearean spelling for a clock or watch that sounds a warning bell at a pre-set time. Alarum mechanisms are found on some of the earliest mechanical timekeepers and were being fitted to watches before 1600.
Found on http://www.encyclo.co.uk/local/20450
No exact match found