Tesla's invention is, in essence, a rotating transformer. Its primary windings reside in a stationary steel casing (the stator) and and secondary conductors are attached to an inner shaft (the rotor).

尼古拉·特斯拉发明的感应电动机本质上就是一台旋转变压器——首先在电动机的固定钢制外壳（定子）上安装上绕组，然后将导体安装入内轴（转子）。

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An iron ore shaft in Hebei was constructed by drilling. After removal of steel casing in the shaft, water leaked around the shaft wall.

河北省某铁矿竖井采用钻井法施工，拆除井筒钢护壁后出现井筒漏水。

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