The middle hole is used for cable going through.

中间窗孔供三相电缆穿过。

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The secondary windings and iron core are insulated by epoxy resin. The middle hole is used for busbar going through.

二次绕组及铁芯均浇注在聚脂树脂内，中间窗孔供一次母线穿过。

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Thickness and middle hole must be exactly same as per this sample. Note that these blades are of carbide, not diamond.

厚度和中间窗孔必须完全相同的按这个样品。注意这些叶片是硬质合金，不是钻石。

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A doughnut is a round piece of fried dough with a hole in the middle or filled with cream or jelly.

VOA: special.2010.04.12

The valley looks like a hole in the middle of the mountains that surround it.

VOA: special.2009.02.16

Scientists observing a distant galaxy catch a black hole in the middle of destroying a planet larger than Jupiter.

FORBES: Scientists Catch A Black Hole Munching On A Giant Planet

Selenium then melted around those drops, which completely dissolved away, leaving a melted selenium ball with a hole in the middle.

ENGADGET: Researchers stumble onto 'lava' generated quantum dots, could power future peripherals

Fomalhaut's disc has a hole in the middle, of the kind that any planets around the star would have swept out.

ECONOMIST: Dusty genesis