火數、瓦(watt,或者叫瓦特)係國際單位制嘅功率計量單位,符號係W。一瓦等如指每秒1焦耳嘅能量速率。
舊時十瓦寫做瓧,百瓦寫做瓸,千瓦寫做瓩,分瓦(十分一瓦)寫做瓰,厘瓦(百分一瓦)寫做甅或者瓼,毫瓦(千分一瓦)寫做瓱。呢啲寫法係來自日本明治時代,之不過呢個瓦係瓦蘭姆,係日本音譯gramme,家下中文克咁解,係重量單位。之後中國引進呢種寫法,變咗係功率單位。
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