_=118. Questions.=_ Will two pieces of electrified glass repel each other? Arrange an experiment to show whether you are right or not. Is the charge on the glass exactly like that on the ebonite? Do you know how to find out? =EXPERIMENT 60. To compare the electrification produced by ebonite and flannel with that produced by glass and silk.= _Apparatus._ The support (see § 109); wire swing, W S (No. 37); ebonite rod, etc., of Exp. 55 (Fig. 29); the glass, G, and silk of Exp. 59. =119. Directions.= (A) Electrify E R, and place it in W S, Fig. 29. (B) Bring the uncharged glass near E R, noting the action of E R. (C) Heat and electrify G; bring it near E R, and carefully note whether the attraction between them is stronger or weaker than before, or whether they repel each other. _=120. Discussion.=_ We know that the glass was electrified, because it lifted tissue-paper; hence, its charge was not of the same kind as that on the ebonite. Had the electrifications been exactly alike, we should have had a repulsion (Exps. 56, 57, 58). The exact difference between these two kinds of electrifications is not known. It has been agreed, for convenience, to call that produced by glass and silk a _positive_ electrification. With ebonite and flannel a _negative_ electrification is produced. The sign + is generally written for the word positive, and - for negative. These signs indicate _kind_, and not more or less, as in arithmetic. _=121. Laws.=_ We have learned from the experiments these facts, which are called _laws_: (1) Charges of the same kind repel each other; (2) charges of unlike kinds attract each other; (3) either kind of a charge attracts, and is attracted by a neutral body. CHAPTER VII. INSULATORS AND CONDUCTORS. =EXPERIMENT 61. To study insulators.= _Apparatus._ Ebonite rod, E R (No. 28); flannel cloth, F C (No. 30); tissue-paper, T P (No. 31). =122. Directions.= (A) Holding one end of E R in the hand, charge the other end by rubbing it with F C. (B) With bits of the T P test each end of E R for a charge, and compare the results. =EXPERIMENT 62. To study insulators.= _Apparatus._ The ebonite sheet, E S (No. 26); flannel cloth, F C (No. 30). =123. Directions.= (A) Thoroughly electrify E S (Exp. 51, D), then lift and hold it in the air, as in Fig. 28. (B) By moving your rounded knuckle about near the surface of E S, see if you can get more than one spark from it. =EXPERIMENT 63. To study insulators.= _Apparatus._ A hard-rubber comb (not furnished); flannel cloth, F C (No. 30); dull pointed nail (No. 19). =124. Directions.= (A) Electrify the comb with F C. (B) Move the nail along near the teeth of the comb, and listen carefully.