The species most common are various root rubbers, notably the Carpodinus chylorrhiza.
It passes between two rubbers made of leather, and is partly covered with two silk aprons which extend over quadrants of its surface.
The glass is excited positively by friction with the rubbers, and the charge is drawn off by the action of the points which, when acted upon inductively, discharge negative electricity against it.
It was found that the machine acted better if the rubbers were covered with bisulphide of tin or with F.
Tin amalgam is used for "silvering" mirrors, gold and silver amalgam in gilding and silvering, cadmium and copper amalgam in dentistry, and an amalgam of zinc and tin for the rubbers of electrical machines; the zinc plates of electric batteries are amalgamated in order to reduce polarization.