Page:Popular Science Monthly Volume 9.djvu/182

162 straw-index I I', shown in Fig. 13. By its means you immediately see both the effect of the first induction and the consequence of touching any part of the system with the finger. The plate N rests over the ball or turnip T, the position of the straw-index being that shown by the dots. Bring the rubbed tube near T': the end N of the index immediately descends and the other end rises along the graduated scale. Remove the glass rod; the index I I'  immediately falls. Practise this approach and withdrawal, and observe how promptly the index declares the induction and recomposition of the fluids.

While the tube is near T, and the end N of the index is attracted, let T'  be touched by the finger. The end N is immediately liberated, for the electricity which pulled it down escapes along the chain and through the finger to the earth. Now remove your excited tube. The captive negative electricity diffuses itself over both balls, and the index is again attracted.

Instead of the chain you may interpose between the balls one hundred feet of wire supported by silk loops. This is done in Fig. 14, which shows the wire w supported by the silk strings S S S, and where, for the ball or turnip, the cylinder C, on a glass support G, is substituted. Every approach and withdrawal of the rubbed glass tube R is followed obediently by the corresponding motion of the index.



Or, substituting a carrot, a cucumber, or other elongated conductor for the ball T', Fig. 12, you cause your rubbed glass tube to act upon a greater extent of surface. You thus decompose more electricity and produce a greater attraction.

Repeat here an experiment, first made by a great electrician named Æpinus. I wish you to make these grand old experiments. Support an elongated metal conductor, or one formed of wood coated with tin-foil—even a carrot, cucumber, or parsnip, so that it will be