EXPERIMENT 66. To study the effect of moisture upon an insulator.
Apparatus. Same as for [Exp. 65], with the exception of the copper wire; this is to be replaced by a dry silk thread about 2 feet (60 cm.) long (No. 33).
135. Directions. (A) See if a charge can be sent through the thread, in the same manner as it was through the copper. Is dry silk a conductor?
(B) Thoroughly wet the thread, being careful not to wet the rubber band insulator ([Fig. 33]); see if wet silk is a conductor.
136. Discussion. Dry silk is an insulator, while wet silk is a good conductor of static electricity. It is the water, however, which really does the conducting. Even small amounts of moisture on glass, or other insulators, will allow the charge to escape. Glass collects much moisture from the air. Do you now see why it is necessary, to get good results, to have the paper, glass, etc., hot before electrifying them?
EXPERIMENT 67. To test the effects of moisture upon bodies to be electrified.
Apparatus. Two pieces of newspaper, each about 4 in. (10 cm.) square.
137. Directions. (A) Heat one piece to make it thoroughly dry, and leave the other cold.
(B) Stroke each, say 10 times, with your hand, pressing them upon the table; then place them upon the wall at the same time, being careful not to let them touch your clothing. See which will cling to the wall the longer.