CHAPTER IX.
INDUCED ELECTRIFICATION.
_=156. Electric Field; Lines of Force.=_ In our study of magnetism you learned that a magnet can act through the air, and induce a piece of iron to become a magnet. You saw how the iron filings arranged themselves around the magnet, showing that the lines of force reached out from the poles in a very peculiar manner. There is an _electric field_ all around a charged conductor, just as there is a magnetic field about a magnet. The lines of force in the electric field pass from the positively charged body to the negatively charged one, or to some neutral one, which, you will soon see, is practically the same thing. When the positively charged electrophorus cover is held above the negatively charged ebonite sheet, a very strong electric field exists between them.
=157. Note.= You have seen that we can _charge_ an insulated conductor by _touching_ it with the charged cover, or by allowing a spark to pass to the conductor. What effect, if any, has a charged body upon an insulated conductor _before_ they touch each other, and before any spark passes to the conductor?
=EXPERIMENT 75. To study electric induction.=
_Apparatus._ Fig. 42. The insulating table, I T (for details see Exp. 64); tin box, T B (No. 47, Fig. 42); moist cotton thread, C T; the electrophorus (Exp. 68); tie C T around one end of the closed T B, and leave the ends of C T long enough to hang down over the end. Place a match on each side of T B to keep it from rolling.
=158. Directions.= _Part 1._--(A) Pass a spark from the charged E C to T B, and note the action of the thread, which will be our electroscope. Remove E C.
(B) Touch the charged T B with the finger, watching C T.
_Part 2._ (C) Bring the re-charged E C near the neutral T B, and parallel to its end surface; but keep them at least an inch apart, so that a spark cannot pass. Watch C T.
(D) Withdraw E C, and try to explain the action of C T.
_=159. Electric Polarization; Theory of Induction.=_ This experiment should remind the student of Exp. 24, in magnetism, in which a piece of soft iron was magnetized by the inductive action of a magnet. The soft iron was in a magnetic field; it became polarized. Is it possible that the box, T B, was polarized, being in the electric field of E C?
We know, by the action of C T (Fig. 42), that the top end of T B was charged while E C was in place. The charge was not conducted.
You know, from previous experiments, that + and - electrifications rush together whenever possible. Why can we not suppose that a neutral body, like the box at the start, contains an equal amount of both kinds, and that these different electrifications have already rushed together?
If you imagine a small army of positive soldiers struggling, "man to man," with the same number of equally strong negative soldiers, you can readily see that one-half of them can hold the other half from running away. A body remains neutral, then, according to this idea, as long as it has an equal quantity of the two opposite kinds of electrification. (See Theories, § 145, 146.)
As soon as the positively charged E C was brought near T B, it destroyed the neutrality of T B, by pulling at its - electrification, and by pushing back its + electrification to the top end and into C T. We say that the charged E C produced a separation of the combined electrifications of T B by _induction_, and not by contact. As soon as the inductive action of E C was removed, T B became neutral again.
=160. Note.= Figs. 43 and 44 may aid the student. In Fig. 43, T B is supposed to be neutral. The "double sign" means that the + and - electrifications are united; and, as there are an equal number of both kinds, none are left free to tell the tale. Fig. 44 shows what happens when the + E C is near.
What would happen if we could cut into T B at the middle with an insulated knife while it is polarized by E C?
=EXPERIMENT 76. To learn how to charge a body by induction.=
_Apparatus._ Fig. 42, same as in Exp. 75.
=161. Directions.= (A) Bring the charged E C within an inch of the bottom of T B, and as soon as C T is repelled, showing that T B is polarized (Exp. 75), touch T B with your finger; then remove your finger while you still hold E C in place.
(B) Withdraw E C and its inductive action. Explain the motions of C T during the experiment. Is it still repelled by T B after E C is removed?
_=162. Free and Bound Electrifications.=_ As explained in Exp. 75, and as shown in Fig. 44, T B became polarized. The - electrification was drawn towards E C; it was held or _bound_ there as long as E C was near. The + was actually repelled by E C, and it was _free_ to escape through your arm as soon as T B was touched, leaving the top end of T B neutral. As soon as E C was removed, the - electrification, no longer held by E C, spread all over T B and on to C T. T B was _charged by induction_. It was charged negatively by driving out + electrification.
=EXPERIMENT 77. To show that a neutral body is polarized before it is attracted by a charged one.=
_Apparatus._ The electrophorus (Exp. 68); dry tissue-paper, T P. Cut out 2 pieces of T P, each about 1/4 inch square.
=163. Directions.= (A) Place the bits of dry T P upon a board or table, and convince yourself that they are attracted equally by the charged E C.
(B) Slightly moisten one piece of T P only. See if one is attracted by E C more readily than the other.
_=164. Polarization Precedes Attraction.=_ Dry tissue-paper is not a good conductor; you have seen (Exp. 52) that it can be electrified, which indicates that it is at least a partial insulator. Insulators are not easily polarized. (Why?) Even if the pieces of T P were polarized, the opposite electrifications were so near each other that the attraction of E C for the - was nearly overcome by the repulsion for the +; the result being that T P was not strongly attracted by E C until the + had a chance to escape. The moist tissue-paper allowed its + to escape more quickly than the dry piece. A conductor is attracted by a charged body more strongly than an insulator, because the latter is not easily polarized. A neutral body, then, is really no longer neutral when it is in the electric field. _Polarization precedes attraction._
=EXPERIMENT 78. To find whether electric induction will act through an insulator.=
_Apparatus._ Small bits of carbon (Exp. 58); bits of moist tissue-paper, T P; one-half of the flat box, T F B (No. 41); sheet of glass, G (No. 38); electrophorus (Exp. 68). Place the carbon and T P into T F B (Fig. 45), and cover with the glass.
=165. Directions.= (A) Charge the electrophorus cover, E C (Exp. 68), move it about a little above the glass, and see if the carbon, etc., are attracted.
_=166. Dielectrics.=_ The carbon must have been polarized and attracted _through_ the glass. You saw, Exp. 7, that the lines of magnetic force could penetrate and act through paper, glass, etc.; it is now evident that the electric field is not easily fenced in, even by an insulator. Substances, like the glass, which allow this inductive influence to act through them, are called _dielectrics_.
=EXPERIMENT 79. To find whether a polarized conductor can act inductively upon another conductor.=
_Apparatus._ Fig. 46. Insulating table, I T (for details see Exp. 64); ebonite sheet, E S (No. 27); flat box complete F B (Nos. 40, 41); sheet of glass, G (No. 38); small piece of slightly moist tissue-paper, T P; charged electrophorus cover, E C. Arrange as shown.
=167. Directions.= (A) Hold E C, charged, near and under I T, then bring your finger, F, near T P. Explain the action of T P.
_=168. Successive Induction.=_ The inductive influence of E C first polarized I T; this acted through the dielectric, E S, and polarized F B, which, in turn, polarized T P through the second dielectric, G. This induction after induction is called _successive induction_.
_=169. Inductive Capacity.=_ Dielectrics are insulators. Two substances may be equally good insulators, that is, they may equally well resist the _spread_ of electrification _over_ their surfaces, or the _flow_ of the electric current _through_ them, while one may be, nevertheless, a better _dielectric_ than the other. The better the dielectric, the easier it is for the electric field to polarize a conductor placed beyond the dielectric. A good dielectric is said to have a high _inductive power or capacity_. Glass is about 3 times as good a dielectric as dry air; and as the latter (under certain conditions) is taken as the standard, or as unity, we may say that the _specific inductive capacity_ of glass is about 3.
=EXPERIMENT 80. To study the action of the electrophorus.=
_Apparatus._ The electrophorus (Exp. 68); small bits of moist tissue-paper, T P.
=170. Directions.= (A) Thoroughly electrify E S, Fig. 34, and place E C upon it by its handle, E R.
(B) Touch E C, as directed in Exp. 68, and listen for a small spark which should pass from E C to your finger.
(C) Again, place a little piece of T P upon E C before lowering it upon E S. Do not touch E C, but bring your finger near T P. What does T P do? Now, touch E C and see, when you bring your finger near it, if T P acts as it did before.
(D) Again, place several pieces of T P upon E C (E S being thoroughly charged); touch E C, then lift it by its handle. Note action of T P, which should be slightly moist.
_=171. Discussion.=_ The electrification upon the ebonite is negative (Exp. 60). Although E S and E C (Fig. 34) seem quite smooth, there are many little hills, valleys, and air-spaces between them, which keep them from touching each other perfectly. The ebonite has the electric field at the start, and it really acts across these minute air-spaces _by induction_ (Exp. 75), and polarizes E C. The air-spaces form the dielectric (Exp. 78). The - electrification of E C being repelled by the - of E S, it is driven to the top of E C, while the + is drawn to the bottom. This + is kept from rushing to the - of E S by the air dielectric, and because E S is a non-conductor. By touching E C the free - escapes to the earth, leaving E C _positively_ charged when it is lifted.
=172. Details of Action.= The different steps in the action of the electrophorus are shown graphically in Figs. 47 to 51. Fig. 47 shows E S negatively charged. E C is neutral at first, Fig. 48; that is, it is supposed to contain both + and -, as shown by the "double sign" (§ 160). Fig. 49 shows that E C has been polarized by the inductive action of E S. The repelled - escapes to the finger (this escaping is what gave the small spark to the finger and charged the T P in the last experiment), leaving the top uncharged, while the + is _bound_ (Fig. 50). As soon as E C is lifted (Fig. 51) the + spreads all over E C, which is then charged. The +, upon going to the top, charged the pieces of T P (Exp. 80, D), causing them to be repelled. The charge of - upon E S has not been removed, so the operation may be repeated many times before E S must be again electrified.
The - electrification on the ebonite acts inductively through E S, drawing up + electrification from the earth. To make this action easier a "sole," or metal conductor, is often placed under the ebonite.
=EXPERIMENT 81. To see, hear, and feel the results of inductive influence and polarization.=
_Apparatus._ Ebonite sheet, E S (No. 26); insulating table, I T; flannel cloth, F C.
=173. Directions.= (A) Thoroughly charge E S with F C. With the right hand bring E S near and parallel to the top surface of I T, but do not let them touch each other.
(B) Remove E S, then touch I T to see if it is charged.
(C) Repeat (A), and while you hold E S about 1/2 inch from I T, their flat surfaces being parallel, touch I T. Watch for any sparks, and note any peculiar actions of E S.
(D) Remove your finger from I T, then withdraw E S; finally touch I T with your knuckle.
_=174. Discussion.=_ This apparatus is really the electrophorus upside down. It shows very clearly (1) the escape of the - electrification from I T, by the spark; (2) that the attraction between I T and E S is much greater than before, when this - is removed; and (3) it shows the different steps of the inducing and charging process, as described in Exp. 75, and as shown in Figs. 43 and 44.