[电] practical system of electrical units

diàn qì dān yuán de shí yòng xì tǒng ㄉㄧㄢˋ ㄑㄧˋ ㄉㄢ ㄩㄢˊ ㄉㄜ˙ ㄕˊ ㄩㄥˋ ㄒㄧˋ ㄊㄨㄥˇ