Excerpt from John Hopkinson British patent 3,576 - 27th July 1882
"In order to combine the advantages of comparatively high potential in the feeding conductors with lower potential in the houses in which electricity is used, I make use of a system of 3
conductors in combination with 2 dynamo machines coupled in series. This system is shewn diagrammatically where AB represents the 2 dynamos, CD and EF represent the extreme conductors and GH
represents the intermediate conductor. The positive pole of the dynamo machine A is connected to the CD and its negative pole to the conductor GH. On the other hand the positive pole of the
dynamo machine B is connected to the conductor GH and its negative pole to the conductor EF".