main·spring /ˈmenˌsprɪŋ/
主發條;主要原因
main·spring n. The principal or most important spring in a piece of mechanism, especially the moving spring of a watch or clock or the spring in a gunlock which impels the hammer. Hence: (Fig.) The chief or most powerful motive; the efficient cause of action; as, the mainspring of action.
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mainspring
n : the most important spring in a mechanical device (especially
a clock or watch); as it uncoils it drives the mechanism