Two Leyden jars are next required, of opposite influences. To effect this, charge one by the ordinary method of touching its terminal several times with an electrophorus cover, being careful on each occasion to “earth” the outer surface, whilst the other condenser is charged by presenting its outer surface to the electrophorus cover, and each time touching the knob terminal with the finger, i.e. “earthing” it.
By this method, whilst the charge at the knob of one Leyden jar is positive, that at the terminal of the other is negative. Place these two instruments at equal distances on either side of the “swing” support, and at such an elevation that the pith figure may just touch the brass knobs.
Then, if the swing be started going, it should continue to oscillate for a considerable time. As the pith figure approaches, say, the positive knob, it acquires a negative charge and is thereby attracted. Directly it touches, however, this negative state is counteracted, a positive charge supersedes it, and repulsion between pith figure and positive knob ensues. The pith doll then swings over towards the negative knob and—being positively influenced—is of course attracted until contact takes place. Then negative supersedes positive in the pith figure, repulsion again results, and a similar cycle of changes is repeated. Thus the figure continues to oscillate until the charges of the jars have been exhausted or have leaked away.
17. In the same manner as the alternate attraction and repulsion of a light body has been used to maintain oscillation, so similarly an electrostatic motor may be constructed embodying the same principles. Cut out a 16-pointed star of cardboard, 8-inch diameter, and, having glued a 1⁄4-inch slice of cork in the middle of each side (A, [Fig. 12]), pass a knitting-needle through the star’s center at right angles (B, [Fig. 12]). Both ends of this spindle rest in conical cavities (C, [Fig. 12]), which may be punched or drilled in small brass discs to act as bearings, and the apparatus is mounted as shown.
Fig. 12.—Electrostatic motor.
It is important that the cardboard should be perfectly dry, and with this end in view it may well have previously received a saturation in paraffin wax. When set hard, stick a brass pin bent at right angles into each point of the star, and set Leyden jars of different charges beneath any two diametrically opposite points. The motor, being given a start, should continue to revolve for some time owing to the succeeding attraction and repulsion of the metal pins.
The following description is of a small machine which may be fairly easily constructed, and when in good working order will give far better results than the electrophorus. [Fig. 13] shows the reference letters and general arrangement. Two cork bungs (A, [Fig. 13]), previously saturated with paraffin wax or shellac varnish, are fixed into the ends of a cylindrical lamp chimney, and through a hole in the exact center of each passes a wooden rod (B, [Fig. 13]), which must be glued in place. Each end is supported in a wooden bearing (C, [Fig. 13]), black lead being used as a lubricant, and to the protruding one a small handle (D, [Fig. 13]) is fitted, wherewith the glass may be revolved. Next a long pad of wash leather (E, [Fig. 13]), stuffed with horse-hair, is fixed so as to press gently against the cylinder, whilst a silk flap (F, [Fig. 13]) about 2 inches wide hangs over the top surface of the glass. A brass chain (G, [Fig. 13]) is also fixed to the wash-leather cushion and, in the ordinary way, connects to “earth” either direct or through the operator. The wash-leather cushion is well black-leaded where it touches the glass.
Fig. 13.—Frictional machine (more useful than the electrophorus for producing electric charges).