交流电是其方向周期地变化的电流。

Alternating current is the current whose direction changes periodically.

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它们与SCM系统联合，自动地观察代码存储库，并周期地或在有任何变更的时候构建系统(包括运行测试和打包工件)。

These work in conjunction with SCM systems to automatically watch the code repository and build the system (including running tests and packaging artifacts) periodically or whenever anything changes.

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它可以比较粗略地描述成是发生在固定周期（这个周期通常是一年一次）的包含某一个物种很多成员的集体旅行，并且只有在长途旅行后才有回报。

It can loosely be described as travel that takes place at regular intervals—often in an annual cycle—that may involve many members of a species, and is rewarded only after a long journey.

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And in contrast, in the lower left hand part of the periodic table, these 2 quantities are low, so also what we're going to see is low electronegativity.

相反地，在周期表的左下部分，这两个量的值都很小，因此我们会看到这里的电负性也很低。

麻省理工公开课 - 化学原理课程节选

Depending on what kind of chemistry you go in to, you might accidentally memorize parts of the table, which is fine, but what you really want to know how to do is know how to use the periodic table.

取决于你今后从事哪个方向的化学研究，你可能会不经意地记住周期表的某一部分，这就够了，但是要注意,你真正需要知道的是如何使用元素周期表。

麻省理工公开课 - 化学原理课程节选

And so here is the Periodic Table in its glory today following on Mendeleev's scheme.

所以，这是周期表,今天非常光荣地跟在门捷列夫的图表之后。

麻省理工公开课 - 固态化学导论课程节选