介绍了电路图中的元器件的作用和符号。一张电路图通常有几十乃至几百个元器件，它们的连线纵横交叉，形式变化多端，初学者往往不知道该从什么地方开始， 怎样才能读懂它-Describes the role of components in the schematic and symbol. A circuit is usually hundreds or even dozens of components, their vertical and horizontal cross-connect, form changing, beginners often do not know where to start, how to read it