_=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.
_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.
_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.
_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.
_=125. Discussion of Experiments 61, 62, 63; Insulators.=_ In 61 the electrification remained at one end of the rod. In 62 and 63 the sparks showed that all parts of the ebonite were not discharged at the same time. A substance, like ebonite, which will not allow electrification to pass from one part of it to another, is called an _insulator_. Silk and glass are also insulators. Do you now see why a silk thread was used to make the carbon electroscope?
Why do they fasten telegraph wires to glass insulators?