One last piece of apparatus—the simplest in construction—remains to be made. Bend a piece of stout brass wire into a semicircle (A, [Fig. 7]); then, having flattened it at the middle, pass through a hole at the end of a wooden handle (B, [Fig. 7]), and fix in position by means of a tiny glued wedge (C, [Fig. 7]). Next twist the rod at both ends into loops, and wrap tinfoil round so as to form metal knobs (D, [Fig. 7]). Brass balls may be used instead. This arrangement is now complete, and we may commence experimenting.
11. Twist a stout piece of copper wire surmounted by a brass or tinfoil ball round the outside coating of a Leyden jar (A, [Fig. 8]), so that the two knobs are about 3 inches apart. Suspend a pith ball (B, [Fig. 8]), made from dried wood pith, by silk thread (C, [Fig. 8]) from some support above, so that it hangs normally midway between the two knobs.
Next excite the resin surface of the electrophorus by rubbing with a dry rabbit’s fur, muff, or flannel; and, holding the cover of this instrument by the handle’s extreme end, place it upon the resin. Remove immediately and bring near to the knob, which connects with the interior of the Leyden condenser, at the same time touching its outer tinfoil surface with the finger. A spark should pass between the electrophorus cover and the Leyden jar, whereupon, if the former and finger be removed, and the pith ball allowed to drop into position, this will oscillate violently to and fro between the knobs.
Fig. 8.—Oscillating pith ball.
Fig. 9.—Sparks from discharger and Leyden jar.
12. Construct a Leyden jar with knob terminal similar to that previously described, but cover part way up the outside with gold paint (A, [Fig. 9]), instead of tinfoil. Charge this condenser by means of the electrophorus cover, as described in [experiment 11], and if sufficient induction does not take place with one contact, replace the metal disc on the resin and pass a spark to the Leyden condenser knob several times, taking care to touch the gold paint coating with the finger on each occasion. If now the semicircular discharger, held by the wood handle, be brought so that one knob touches that of the Leyden condenser whilst the other moves over the gold-paint surface, long series of sparks, differing greatly in appearance from any previously produced, will pass between the gilt and the discharger.
13. Excite a stick of sealing-wax by rubbing with flannel, and bring it against a pith ball suspended by silk. The sealing-wax was negatively charged, and the pith ball is now in a like condition. To prove this, approach the flannel which excited the wax, and had consequently acquired a positive charge; inasmuch as the pith ball is immediately attracted and we know that positively excited bodies have an affinity for those negatively affected, the pith ball must be of the latter character.
If, further, a glass rod be excited by rubbing with silk and brought near to the negative pith ball, the latter will again be attracted, showing the glass to be positively charged.