CHAPTER VIII.

CHARGING AND DISCHARGING CONDUCTORS.

_=138. The Electrophorus.=_ While the ebonite sheet alone, or a good hard-rubber comb, may be used for many experiments in frictional electricity, the sparks produced are small, and the ebonite has to be electrified as often as it is discharged. To obtain real good sparks, and to avoid this continual rubbing, the student should be provided with an _e-lec-troph'-o-rus_. This is, really, a simple, cheap, and efficient frictional electric machine. An electrophorus consists of 2 insulators and 1 conductor--that is, of 3 parts: (1) insulating handle, (2) cover, and (3) a plate or base of insulating material.

=139. Our Electrophorus= is shown in Fig. 34. For the insulating _handle_ use the ebonite rod, E R (No. 28); for the _plate_, use the ebonite sheet, E S (No. 26). The _electrophorus cover_, E C (No. 42), furnished, is a tin box with a fancy top. A hole has been punched in the center of its top, and into the hole has been riveted a short tube, so that the handle, E R, can be firmly held. The hole has been made a little larger than E R for convenience. To make E R fit tightly in the hole, so that you can lift E C, wrap a small piece of paper around the end of E R before pushing it into the hole. You can easily find out how much paper to use to make a good fit. With a knife cut away all loose points of paper that stick out of the hole around E R; this is _important_. The top and bottom of E C should be pressed firmly together.

First learn how to use the electrophorus. With the large amount of electrification produced we can then find out how it works.

=EXPERIMENT 68. To learn how to use the electrophorus.=

_Apparatus._ Shown in Figs. 34, 35. _Do not fail to read_ § 139.

=140. Directions.= (A) Place E S upon a _flat_, uncovered, wooden table, and rub it _vigorously_ for a _minute_ with the _warm_ flannel, F C, to thoroughly charge it. Do not let E S slide about, and do not lift it from the table.

(B) With the right hand grasp E R at its extreme end, and place E C upon E S.

(C) Touch E C for an instant with a finger of your left hand (Fig. 35).

(D) Remove your finger entirely from E C, then lift E C by its insulating handle, E R, at the same time holding E S down to the table, if it tries to follow E C.

(E) Bring your left hand near E C (Fig. 36). You should get a good spark from E C.

(F) It is not necessary to immediately rub E S again. You have discharged E C by taking a spark from it. To _recharge_ it, simply place it upon E S again; let it remain there while you count 5; touch it as before, and then lift by E R.

=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.

=EXPERIMENT 70. To study potential; electro-motive force.=

_Apparatus._ The insulating table, I T, Fig. 38. (For details see Exp. 64; the electrophorus Exp. 68).

=143. Directions.= (A) Pass a spark from the thoroughly charged E C (Exp. 68) to I T.

(B) Recharge E C, and see how many times I T will take good sparks from it, and note the relative sizes of the sparks.

(C) As soon as I T refuses to take more sparks from E C, touch E C to see if it is completely discharged.

(D) Touch I T.

_=144. Pressure; Potential; Electro-motive Force.=_ Water runs down hill. It always tries to run from a high place to a lower one. Electrification acts very much like water in this respect. We say that water has a _pressure_, or a _head_ of so many feet. In speaking of a charge, we say that it has a _potential, or an electro-motive force_. Water may have a high or low pressure, and a charge may have a high or low potential. The greater the pressure of water, the harder it tries to break away and get somewhere; the greater the potential of a charge, the farther it will jump to your hand.

_=144a. Current; Spark.=_ Electrification will easily pass from a place of high potential to one of low potential through a conductor, and when it _passes_ we say we have an _electric current_, or a _current of electricity_. Water has no desire to flow on a dead level, and the electric current does not care to flow between two places of equal potential. The potential of the earth and of all neutral bodies is zero; that is, they have no charge, no potential; so it is very easy for a charge to escape into the earth.

Dry air is a pretty good insulator, but when the attraction between a charged and a neutral body gets great enough, the spark rips right through the air. Benjamin Franklin proved by experiment that lightning is caused by the electrification in the clouds and air. (See Atmospheric Electricity.)

=145. Theories about Electrifications.= _The "One-Fluid" Theory_ suggests that neutral bodies have a certain amount of electrification, and that they have a certain potential called zero potential. If the potential of a body becomes greater than that of the earth, the body is said to be positively electrified; if the potential of the body is less than that of the earth, it is said to be negatively electrified. If we fill a bottle with sea water, we have a great deal of water when we compare it with the bottle, but a very little water when we compare it with the sea. The earth is so large that small amounts of electrification taken from it or added to it do not affect its potential to any extent.

=146.= _The "Two-Fluid" Theory_ suggests that there are two absolutely different kinds of electrification, one called positive (+), and the other negative (-). When these two are equal in quantity, the body is said to be neutral. If the body contains more + than -, the body is said to be charged positively.

It is evident then, if the two-fluid theory be accepted, that no matter how strongly a body is charged positively there must be in it _some_ negative electrification; that is, we may charge a neutral body + by adding + electrification to it, or by taking - electrification from it. There must always be, then, some + and - electrifications in a body.

These theories do not require much consideration by the student of elementary electricity. The best thing he can do is to learn what electricity can do, and how it can be used.

=EXPERIMENT 71. To study some methods of discharging an electrified body.=

_Apparatus._ The electrophorus (Exp. 68); an ordinary pin (Fig. 39).

=147. Note.= You have seen sparks pass from E C to your rounded knuckle, and to other conductors. In all of these cases the discharge was _sudden_, one spark doing the work. Can we _slowly_ discharge E C, or discharge it without sounds?

=148. Directions.= (A) Thoroughly charge E C, and test it with your knuckle to be sure that it is working properly.

(B) Charge E C again; hold the pin in your left hand (Fig. 39), and _slowly_ bring its _head_ toward E C; listen for sparks.

(C) Recharge E C, and bring the _point_ of the pin slowly toward it. Touch E C to see whether it has been discharged or not.

_=149. Disruptive, Conductive, and Convective Discharges.=_ Sudden discharges, accompanied by bright sparks, are said to be _disruptive_. When the electrification is continuously carried away by a conductor, there is a _conductive_ discharge. There is a _convective_ discharge when the electrification escapes from points into the air. (See § 155.) The nature of the discharge depends upon the potential of the charge, upon the nature of the charged conductor, and upon the nature of the surrounding air and objects. Convective discharges are often _silent_, as in Exp. 71 (C). In this case, electrification passed from the earth through the pin-point to the cover to neutralize it. (See Induced Electricity.)

=EXPERIMENT 72. To study intermittent or step-by-step discharges.=

_Apparatus._ Electrophorus (Exp. 68); carbon electroscope (§ 114), (Exp. 58).

=150. Directions.= (A) Charge E C, then hold your hand on one side of the carbon (Fig. 40), and hold E C upon the opposite side. What should the carbon do?

_=151. Discussion.=_ The carbon and E C were insulated, while the hand was "grounded"--that is, it was connected with the earth. Carbon is a good conductor; it may be quickly charged and discharged.

=EXPERIMENT 73. To ascertain the location of the charge upon an electrified conductor.=

_Apparatus._ The electrophorus (Exp. 68); the insulating table, I T (Exp. 64); the tin box, T B (No. 47), Fig. 41; a piece of moist cotton thread, C T, 5 or 6 in. long, bent double, and hung over the edge of the open box, T B. One-half of C T should be inside of T B, which, in turn, should stand on I T.

=152. Directions.= (A) Charge E C; pass a spark to T B, and note the action of both parts of C T.

_=153. Hollow and Solid Conductors.=_ The moist thread, being a conductor, became charged as well as the box. The electrification seemed to be entirely on the outside of T B. A hollow conductor will hold as large a charge as a solid one having the same amount of surface. This refers to charges of static electricity, not to currents. An electric current passes through the whole substance of a conductor.

=EXPERIMENT 74. To study the effect of points upon a charged conductor.=

_Apparatus._ The electrophorus (Fig. 34); a pin, bent slightly to keep it from rolling.

=154. Directions.= (A) Charge E C; test its charge with your knuckle. Be sure that you get a good spark.

(B) Charge E C again, and hold it by its insulating handle, E R, long enough to count 10 before discharging it with your knuckle. Be sure that it holds its charge during this time.

(C) While E C is upon E S (Fig. 34), lay the bent pin upon E C, so that its point will project into the air. The point should stick out about 1/4 in. from the edge of E C.

(D) Touch E C; raise it by E R; count 10 as before; then test with your knuckle to see if E C is still charged.

_=155. Electric Density; Electric Wind.=_ A charge resides upon the outside of a conductor (Exp. 73), and it continually tries to escape. It seems to pile up at points and corners, and we say that it is denser at such places than at well-rounded parts of a charged conductor. All points and sharp places should be removed from a conductor, if it is desired to keep a charge for any length of time.

Electrification may escape from a point so rapidly that currents are produced in the surrounding air. As the particles of air become charged, they repel each other. The movement of the air particles may be so great that a lighted candle will be affected when placed near the point. This current of air is called _electric wind_.

Electrification easily passes from points, and the electrophorus may be easily and silently discharged by holding a pointed pin near it (Exp. 71, C). Thorns, leaves with sharp edges, etc., have a great effect upon atmospheric electricity. They allow a silent escape of electrification from the earth to neutralize that in the clouds which is opposite in nature. (See Atmospheric Electricity.)