Apparatus. Our electroscope ([Fig. 57], [§ 202]); ebonite rod, E R (No. 28); ebonite sheet, E S (No. 27); glass, G (No. 38); flannel cloth, F C (No. 30).
207. Directions. (A) With the charged E R charge the electroscope negatively by conduction ([Exp. 93]). Note the amount of permanent divergence of the leaves.
(B) Electrify the glass, which will be +, (or use the + E C), and slowly lower it over I T, noting the effect upon L. Raise and lower G or E C several times. Does G, which has an opposite charge to the electroscope, make L diverge more or less?
(C) Discharge the electroscope and recharge as in (A).
(D) Slowly lower the charged E S over I T.
(E) Slowly lower the palm of your hand over I T.
Note. If the + G is brought too near the -ly charged electroscope, L will first collapse and then instantly diverge again with a + charge by contact. The first motions should be observed.
208. Discussion. As a neutral body causes a slight collapse of the leaves, as well as a body charged positively (when the charge in the leaves is -), an increase of divergence is really the only sure test to tell how a body is charged. The - leaves collapse when a + body is brought near I T, because the - in them is drawn up towards the body. The leaves diverge more when a - body is brought near, because the - in I T is repelled into them.
209. The Proof-plane. Since charges of static electricity reside upon the outside of conductors, it is an easy matter to take samples of the electrification. This may be done with a little instrument called a carrier, or proof-plane. It consists of a small conductor with an insulating handle. A ring or coin may be used for the[80] conductor, and a silk thread for the handle. By touching the carrier to any charged body, it, also, becomes charged; and the nature of the charge may be determined by the use of a previously charged leaf electroscope ([Exp. 95]). A delicate gold-leaf electroscope would be ruined by coming in contact with a heavily charged body. The carrier allows a small sample to be tested.