What is the operating principle of a DC machine generator?

When the frame rotates around the axis, its left and right halves cyclically pass around the south or north pole of the magnets. In them, each time there is a change in the direction of the currents to the opposite so that at each pole they flow in one direction. In order to create a direct current in the output circuit, a half-ring is created on the collector assembly for each half of the winding. The brushes adjacent to the ring remove the potential of only their sign: positive or negative.