Bodies at the _same_ potential as the earth, or at zero potential, are also said to be _neutral_. Those positively electrified have a positive potential, those negatively electrified have a negative potential. As in gases, movement always tends from higher pressure (potential) to lower pressure (potential).

[Ill.u.s.tration: FIG. 208.--The metal plate gives the electroscope a greater surface and hence greater capacity.]

=230. Capacity.=--If we have a 100-gallon tank and a 10-gallon tank connected by a pipe both filled with compressed air, the larger tank will contain ten times as much air as the smaller at the _same pressure_ since it has ten times the capacity, or, if the two tanks are separated and the same amount of air is contained in each, the pressure of the air contained in the small tank will be ten times that in the large one.

The _electrical capacity_ of a conductor is in some respects similar to the capacity of a tank for air. Since, however, electrical charges are upon the surface of a body, its capacity depends in part upon the extent of surface. For example, if a charge is taken from a charged rubber rod by a proof plane to an electroscope a certain divergence of the leaves will be noticed. If a circular metal plate several times the diameter of the top of the electroscope is laid upon the latter (see Fig. 208), and a charge equal to that used before is brought to the electroscope, the leaves show less divergence than before, showing that the _same charge gives a lower potential when placed upon a body of greater capacity_.

[Ill.u.s.tration: FIG. 209.--A plate condenser.]

[Ill.u.s.tration: FIG. 210.--A condenser of several plates.]

=231. The electric condenser= is a device having a large electrical capacity consisting of parallel conductors separated by good insulators.

It has been devised to enable one to obtain a large electrical charge upon a body of convenient size. Such an apparatus is of great practical value in many experiments and operations. Its construction involves the principle of electrostatic induction in which a charge of one kind attracts and "holds" strongly a charge of opposite kind near it. In its simplest form it consists of two parallel conductors separated from each other (Fig. 209). The upper plate has been charged negatively. This has given the lower plate a positive charge by induction, since the latter is connected to the earth. These positive and negative charges hold or "bind" each other so that a large quant.i.ty may be acc.u.mulated. To increase the capacity of a condenser, several plates are used connected as in Fig. 210.

It is a curious fact that the kind of insulator between the charged conductors of a condenser affects its capacity. Thus if gla.s.s, paraffine, or beeswax is between the plates instead of air, the plates will "hold" more electricity at the same potential. For this reason condenser plates are often separated by sheets of gla.s.s, paraffined paper, or mica.

[Ill.u.s.tration: FIG. 211.--A Leyden jar and a discharger.]

=232. The Leyden Jar.=--A convenient form of condenser, used as long ago as 1745, is the Leyden jar. It consists of a gla.s.s jar (Fig. 211) coated part way up, inside and out, with tinfoil. The inner coating is connected by a chain to a k.n.o.b at the top. The Leyden jar is charged by connecting the outer coating to the earth while to the inner coating is given a charge of either kind of electricity. The other kind of charge is developed by induction upon the outer coating, and each charge binds the other. To discharge a jar, a conductor, as a wire, is connected first to the outer coating and held there while the other end is brought to the k.n.o.b at the top. A bright spark is produced when the two charges combine. It is best not to let the discharge from the jar pa.s.s through the body unless one is certain that only a very small charge is present.

=233. Oscillatory Discharge.=--The discharge from a Leyden jar is an interesting phenomenon. The rush of electricity from one coat to the other does not stop when the two coats are exactly neutralized but continues until the two plates are charged just oppositely to their condition at first, then a rush of electricity in the opposite direction occurs. This alternation continues several times and const.i.tutes what is called the _oscillatory discharge_. (See Fig. 414.) This oscillatory discharge sets up waves in the ether. These are called _Hertzian_ waves in honor of their discoverer, Heinrich Hertz. They are the ether waves used in wireless telegraphy. A _lightning flash_ has been shown by photographs and by other means to be oscillatory. This fact supports the idea that the electrical conditions just preceding the stroke of lightning reproduce a condenser on a large scale. The charged cloud is the upper charged plate, the earth beneath, charged by induction from the cloud, is the lower charged plate, while the air between is the insulator or _dielectric_ as it is sometimes called.

Important Topics

1. Potential: high, low, zero, positive, negative, similar to temperature and air pressure.

2. _Capacity_ affected by (1) area, (2) induction.

3. Condensers, Leyden jar, parallel plate.

4. Oscillatory discharge, conditions, results.

Exercises

1. Is the air a conductor? Explain.

2. Can the Leyden jar be strongly charged if the outer coat is insulated? Explain.

3. Upon what two conditions does the capacity of a body depend? How in each case?

4. Would a lightning discharge produce wireless waves? Explain.

5. If a sharp tack be dropped point up on the plate of an electroscope the latter is quickly discharged. Explain.

(5) ELECTROSTATIC GENERATORS

=234. Static Electric Machines.=--Many machines have been invented to produce larger quant.i.ties of static electricity than we have used in the experiments previously described. One of the earlier of these was the _plate friction machine_ in which a large circular gla.s.s plate was rotated while a pad of some material was held against it. This machine was capable of producing powerful effects, but it took much work to turn it, and it has been abandoned for a more efficient device, the _static induction machine_.

=235. The electrophorus= is the simplest static induction generator, consisting simply of a flat circular plate of some insulating material, as paraffine, sh.e.l.lac, or rosin contained in a metal pan, and a flat circular metal _disc_ having an insulating handle.

[Ill.u.s.tration: FIG. 212.--An electrophorus.]

_The electrophorus is used as follows_: The plate is first electrified by rubbing or beating with fur or a woolen cloth. The plate will be found to be charged negatively. The metal disc is placed upon the plate by holding the insulating handle. The upper surface of the charged body is slightly uneven so that the disc touches but a few high points. The greater part of the charged surface is separated from the metal disc by air, a good insulator. The charge therefore acts inductively upon the disc _repelling negative_ electricity to the upper surface of the disc, leaving the lower surface charged positively (Fig. 212). If now the finger is touched to the disc the repelled negative charge escapes and the whole disc is left positively charged. The disc is now removed (Fig.

213) and the charge upon it may be tested or used in any desired manner.

The disc may be recharged many times without rubbing the plate again.

[Ill.u.s.tration: FIG. 213.--Electrophorus charged.]

These electrical charges possess _energy_. What is the source of this energy? The answer may be determined by the following experiment. Place the disc upon the charged plate. Touch the disc with the finger to remove the repelled charge. Connect an electroscope to the disc by a fine wire. Nothing appears on the electroscope, since the disc has been connected to the earth, and is therefore at zero potential. If now the disc is lifted slowly, the leaves of the electroscope gradually separate, showing that a charge of electricity appears when the disc is being lifted against the force of attraction between the two charges.

Just as potential energy is developed in a weight when it is lifted against the earth's attraction so electrical energy appears in the disc while it is being separated from the plate. The electrical energy of the charge is therefore due to the work done in separating the two charges.

This electrical energy appears as heat and light, when the disc is discharged. It may be employed to ignite gas, gunpowder, etc.

=236. The Toepler-Holtz Induction Machine.=--This is a type of induction or influence machine that is often used for producing a continuous supply of electricity as in the operation of "X" ray machines, in lecture demonstrations, etc. This machine (Fig. 214) consists of two discs: one fixed, the other mounted so as to revolve. Upon the back of the fixed plate are two sectors of tinfoil which become charged oppositely. Upon the revolving plate are six metallic discs. These discs act like the discs of the electrophorus. They become charged by induction from the charges upon the sectors fastened to the fixed plate.

The brushes held by a rod touch the discs at just the right time to take off the repelled charge. The charges induced upon the discs are taken off by two metal combs whose points are held close to the revolving disc. The Leyden jars a.s.sist in acc.u.mulating a good strong charge before a spark pa.s.ses between the terminal k.n.o.bs. Some machines are built up of several pairs of plates and give correspondingly large amounts of electricity.

[Ill.u.s.tration: FIG. 214.--The Toepler-Holtz induction machine.]

Important Topics

_Static Electric Generators._--(a) plate friction machine, (b) electrophorus, (c) induction or influence machine.

Exercises

1. Potential is similar to what other terms that we have studied?

2. What three electrical phenomena are better understood from a study of the lines of force?

3. How many charges may be produced by an electrophorus before the plate needs to be electrified again? Explain.

4. The static induction machine is often called a "continuous electrophorous." Why?

5. The Leyden jars used with the induction machine cause much brighter sparks to be produced than without them. Explain.

6. With the Leyden jars removed, would the frequency with which the sparks pa.s.s between the k.n.o.bs be increased or decreased? Explain.

7. Mention three likenesses and three differences between magnetism and static electricity.

8. Will you receive a greater shock by touching a k.n.o.b of a charged Leyden jar when it is held in the hand or when it is standing on a sheet of gla.s.s? Explain.

9. In what way may an electric charge be divided into three equal parts?

Review Outline: Magnetism and Static Electricity