CHAPTER XII.
MISCELLANEOUS EXPERIMENTS.
EXPERIMENT 96. To show that friction always produces two kinds of electrifications.
Apparatus. Fig. 58. The carbon electroscope (Exp. 58); flannel cloth, F C, doubled twice to make 4 thicknesses (see Fig. 58); ebonite sheet, E S (No. 26); ebonite rod, E R (No. 28); charged electrophorus cover, E C.
210. Directions. (A) Vigorously rub E S with F C (folded as in Fig. 58). See if you can discover any attraction between them.
(B) Rub E S again, but do not lift F C from it with the hand alone. Slip E R under the top fold in F C (Fig. 58), and lift F C straight up from E S. Do not let F C touch the table or your hand.
(C) See if F C is charged, using 2 or 3 different tests.
(D) Charge the electroscope with F C until the carbon is strongly repelled.
(E) Bring the positively charged E C slowly near the carbon, and note the result.
(F) Slowly bring the negatively charged E S near the carbon that has been charged by contact with F C.
211. Discussion. This experiment showed that while the ebonite was negatively charged, the flannel was positively charged. One kind of electrification is never produced without the other. It can also be shown that the two kinds are equal in amount.
EXPERIMENT 97. To show "successive sparks."
Apparatus. Fig. 59. The electrophorus (Exp. 68); the extra ebonite sheet, E S (No. 27); three coins (marked A, B, C, in Fig. 59). The coins should nearly touch each other, and rest upon E S. A part, only, of the electrophorus cover is shown.
212. Directions. (A) Thoroughly charge the electrophorus cover.
(B) Place your finger upon the coin marked A, to "ground" it, then quickly touch the coin C with the charged cover, at the same time watching for sparks between the coins. If you cannot see the sparks, darken the room a little, and look at the center coin, B, while doing the experiment.
EXPERIMENT 98. To show to the eye the strong attraction between a charged and a neutral body.
Apparatus. The flat box, F B (Nos. 40, 41); the electrophorus (see Exp. 68).
213. Directions. (A) Stand F B upon its edge upon a level table, then bring the charged electrophorus cover near it.
(B) Instead of the above, use light hoops made of paper, eggshells, feathers, sawdust, etc.
EXPERIMENT 99. To feel the strong attraction between a charged and a neutral body.
214. Directions. (A) Hold F B in the left hand, as shown, then slowly bring near it the charged cover, at the same time looking between them so that you can keep them the same distance apart all the way round.
(B) Bring them near enough to allow a spark to pass from E C to F B.
EXPERIMENT 100. The human body a frictional electric machine.
Apparatus. Yourself; a carpet; a room with dry air, easily had on a cold winter's day.
215. Directions. (A) Scrape your feet along upon the carpet, then quickly touch your finger to some conductor, as, for example, a friend's nose.
(B) It is possible to light the gas by the above process. Have a friend turn on the gas just before you bring your finger to the jet, and be sure that the spark from your finger passes through the gas on its way to the conductor, the jet.
(C) Bring your finger quickly near a small piece of tissue paper after you have scraped your feet along to charge your body.
216. Static Electric Machines are used to produce large quantities of static electricity. In the early forms, the electrification was produced by friction. Modern machines depend upon the principle of induction. The electrophorus (Exp. 68) is really a very simple form of induction machine. The potential of these machines is very great, as the spark may jump many inches. Thousands of Galvanic cells would be needed to make a spark an inch long. When the spark passes through the air it meets with an extremely high resistance, as air practically insulates ordinary electricity. This high resistance in the circuit reduces the strength of the current. While the potential is very high, the strength of the current is very low. (See Ohm's Law.)