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Obviously you cannot supply AC electricity to a DC motor and expect it to rotate. You first have to rectify the current – that’s the process of AC to DC conversion. These days rectifiers are made using solid state electronics, and can comfortably fit under the floor of a rail vehicle. Back in the 1890s though, the only option was the Mercury Arc Rectifier, a large, sensitive and vulnerable glass bottle containing, um, mercury. Yikes.