any machine for producing powerful electrical effects. The name is, however, seldom applied to machines depending on magneto-electric principles, but is practically confined to two classes of machines--those which act by friction and those which act by electrostatic induction. The former are called friction machines and the latter influence machines. For many years the former were the only kind known, but they have now been almost superseded by the latter. In friction machines, the electricity is generated by the friction of either a glass cylinder or a circular glass plate against cushions covered with an amalgam of zinc and tin. The positive electricity, which is thus developed on the surface of the glass, is given off to an insulated brass conductor, furnished with teeth like those of a comb the sharp points of which are nearly in contact with the glass. The negative electricity, which is at the same time generated on the cushion, must be provided with some means of escaping, or the action of the machine would soon stop. It is usually allowed to escape to the earth by a brass chain, connected with the cushions; but in some machines a negative conductor, connected with the cushions, is insulated, like the positive conductor, by a glass support. Negative sparks can then be drawn from this conductor at the same time that positive sparks are drawn from the other. Friction machines have been almost entirely replaced by influence machines, and descriptions of these are found in ordinary text-books on physics.