The supporting screw is adjusted so that the point at the end of the staff penetrates slightly into the mercury, almost touching its surface.
At approximately 60 degrees away from the brass column, a lathed wooden column supports a horizontal U-shaped magnet with long, flat arms (length 11.8 cm, width of each arm 1.3 cm, gap 1 cm) which can slide along a guide almost touching the edge of the cup at a distance of one or two millimetres. The overall height of the apparatus is 27 cm.
Without the U-shaped magnet the apparatus substantially reproduces the one invented by Faraday in 1821 and described by Ampère in a paper of the same year in which he presented one of his own design based on the same principle.