CHAPTER XI.

ELECTROSCOPES.

_=200. Electroscopes=_ are instruments to show the presence, relative amount, or kind of electrification on a body. (See Apparatus Book, Chap. XVIII, for Home-Made Electroscopes.) The _carbon electroscope_ has been described (Exp. 58). The _pith-ball electroscope_ is made by using pith from elder, corn-stalk, or milk-weed, in place of the carbon. The _gold-leaf electroscope_ is a very delicate instrument. The gold-leaf is supported, as suggested in Fig. 57, at the lower end of a wire conductor which sticks through and hangs from the cork of a glass jar or flask. To the top end of the wire is soldered a ball or disk. The glass jar insulates the gold-leaf, and keeps it dry and free from dust.

=201. Our Leaf Electroscope= (Fig. 57) is made with aluminum-leaf. Gold-leaf is too delicate for unskilful handling, and aluminum will do for all ordinary experiments. To cut it into any desired shape, place it between two sheets of paper, then cut through paper and all.

=202. Construction.= Bend one leg of a hairpin, H P, as in Fig. 57, and slide it onto I T. Hang a wire, W, or another hair pin straightened, then bent, from the horizontal leg of H P. This is to support the "leaves," L, which are made from a strip of aluminum-leaf about 4 in. long and 3/4 in. wide. Moisten the under side of the horizontal part of W with paste or mucilage; press it upon the middle of the strip laid flat upon the table, and then lift W. The leaves should cling to W. Each leaf should be, then, 2 in. long. They should hang close together when not in use. A large chimney, or fruit-jar, may be used to surround the leaves, and to keep currents of air from them. The leaves should not touch the side of the jar when spread.

=EXPERIMENT 93. To study the leaf electroscope; charging by conduction.=

_Apparatus._ The leaf electroscope (Fig. 57, § 201, 202); ebonite rod, E R (No. 28); flannel cloth, F C (No. 30).

=203. Directions.= (A) Thoroughly charge E R, then scrape it along upon I T, noting the action of the leaves, L.

(B) See if the leaves will remain spread for some time.

(C) Touch I T to discharge it, and note the action of L.

_=204. Discussion.=_ No explanation should be necessary for this. Are the leaves charged alike? As they were charged by contact, is the electrification on them + or -?

=EXPERIMENT 94. To charge the leaf electroscope by induction.=

_Apparatus._ Our electroscope (Fig. 57, § 202); ebonite sheet, E S (No. 27); flannel cloth, F C (No. 30).

=205. Directions.= (A) Charge E S with F C, then hold E S above I T (Fig. 57), their surfaces being kept parallel and about 2 or 3 inches apart. Watch the leaves.

(B) Withdraw E S. Do the leaves remain spread?

(C) Repeat (A), and before removing E S, touch I T.

(D) Remove your finger from I T, then withdraw E S. Do the leaves now remain spread?

_=206. Discussion.=_ The permanent divergence of L was due to a charge given by induction. (Exp. 76.) As E S was -, what was the kind of a charge in L? Did any electrification go to the electroscope from E S? In (C) what became of the charge in L? Explain why the leaves again diverged in (D). The electroscope was charged with + electrification by taking - out of it.

=EXPERIMENT 95. To learn some uses of the electroscope.=

_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 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.