The scale is attached to the bottom end of the shaft; the needle is free to rotate around a horizontal axis since it is housed in a longitudinal aperture of the horizontal part of the staff on the edges of which it lies on a pin passing through its barycentre.
The upper end of the shaft terminates on the outside of the case with a clamping pommel.
A vertical clamp is fixed to one of the feet of the instrument.
When the difference in potential to be measured is applied between the needle and the case, the needle is repulsed by the supporting staff and the value can be read directly on the scale.