[电] characteristic impedance of free space
zì yóu kōng jiān de tè xìng zǔ kàng ㄗˋ ㄧㄡˊ ㄎㄨㄥ ㄐㄧㄢ ㄉㄜ˙ ㄊㄜˋ ㄒㄧㄥˋ ㄗㄨˇ ㄎㄤˋ