This paper first introduces the fundamental knowledge of Electromagnetic valve, such as theory and structure, and introduces the Features of Electromagnetic valve.

本文首先介绍了电磁换向阀的原理、结构等一些基本知识，并且简要概括了电磁换向阀的特点。

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That is why practice is the criterion of truth and why "the standpoint of life, of practice, should be first and fundamental in the theory of knowledge".

所谓实践是真理的标准，所谓“生活、实践底观点，应该是认识论底首先的和基本的观点”，理由就在这个地方。

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In the end of the 19th century, sociology first began to pay close attention to studies of social control, and established the fundamental knowledge system of social control theory.

19世纪末期，社会学首先开始关注社会控制研究，奠定了社会控制理论的基本知识体系。

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