141. Extra Notes. You may repeat the above operation many times. As soon as the sparks begin to get small, electrify E S again. The charge on E C is +, although that on E S is -. You will understand, later, why this is so.
If you do not get a good spark from the electrophorus, read the directions again. The ebonite must be well electrified; the cover must be lifted by the end of its handle; you must touch the cover and withdraw your finger from it before lifting. You must allow the cover to remain upon the ebonite 3 or 4 seconds each time. The board, or table, upon which E S rests, must be flat, and not warped, so that E C will fit down perfectly upon E S.
EXPERIMENT 69. To study "charging by conduction."
Apparatus. [Fig. 37]. To one end of a silk thread, S T, is tied a little bent clamp, B C (No. 46); the other end of S T is tied to the support wire, S W (No. 36); the bottom of the flat box, B F B (No. 40), is supported by B C, and thus insulated from the table and earth; the electrophorus ([Exp. 68]) is also necessary.
142. Directions. (A) Charge E C ([Exp. 68]), and bring it near B F B ([Fig. 37]). Note the spark.
(B) Repeat (A) twice, noting the relative sizes of the sparks. Does B F B continue to be attracted by E C?
(C) Bring your knuckle slowly towards the charged disk, B F B.
