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).