Otto von Guericke, burgomaster of Magdeburg, was the first to invent a machine for exciting the electric power in larger quantities by simply turning a ball of sulphur between the bare hands. Improved by Sir Isaac Newton and others, who employed glass rubbed with silk, it created sparks several inches long. The ordinary frictional machine as now made is illustrated in figure i, where P is a disc of plate glass mounted on a spindle and turned by hand. Rubbers of silk R, smeared with an amalgam of mercury and tin, to increase their efficiency, press the rim of the plate between them as it revolves, and a brass conductor C, insulated on glass posts, is fitted with points like the teeth of a comb, which, as the electrified surface of the plate passes by, collect the electricity and charge the conductor with positive electricity. Machines of this sort have been made with plates 7 feet in diameter, and yielding sparks nearly 2 feet long. The properties of the "electric fire," as it was now called, were chiefly investigated by Dufay. To refine on the primitive experiment let us replace the shreds by a pithball hung from a support by a silk thread, as in figure 2. If we rub the glass rod vigorously with a silk handkerchief and hold it near, the ball will fly toward the rod. Similarly we may rub a stick of sealing wax, a bar of sulphur, indeed, a great variety of substances, and by this easy test we shall find them electrified. Glass rubbed with glass will not show any sign of electrification, nor will wax rubbed on wax; but when the rubber is of a different material to the thing rubbed, we shall find, on using proper precautions, that electricity is developed. In fact, the property which was once thought peculiar to amber is found to belong to all bodies. ANY SUBSTANCE, WHEN RUBBED WITH A DIFFERENT SUBSTANCE, BECOMES ELECTRIFIED. The electricity thus produced is termed frictional electricity. Of course there are some materials, such as amber, glass, and wax, which display the effect much better than others, and hence its original discovery. In dry frosty weather the friction of a tortoise-shell comb will electrify the hair and make it cling to the teeth. Sometimes persons emit sparks in pulling off their flannels or silk stockings. The fur of a cat, or even of a garment, stroked in the dark with a warm dry hand will be seen to glow, and perhaps heard to crackle. During winter a person can electrify himself by shuffling in his slippers over the carpet, and light the gas with a spark from his finger. Glass and sealing-wax are, however, the most convenient means for investigating the electricity of friction.