Electricity for Boys Part 2

[Ill.u.s.tration: _Fig. 13._ MAGNETIZED FIELD]

[Ill.u.s.tration: _Fig. 14._ MAGNETIZED BAR]

In Fig. 13 is shown a piece of wire (A). Let us a.s.sume that a current of electricity is flowing through this wire in the direction of the darts.

What actually takes place is that the electricity extends out beyond the surface of the wire in the form of the closed rings (B). If, now, this wire (A) is wound around an iron core (C, Fig. 14), you will observe that this electric field, as it is called, entirely surrounds the core, or rather, that the core is within the magnetic field or influence of the current flowing through the wire, and the core (C) thereby becomes magnetized, but it is magnetized only when the current pa.s.ses through the wire coil (A).

[Ill.u.s.tration: _Fig. 15._ DIRECTION OF CURRENT]

From the foregoing, it will be understood that a wire carrying a current of electricity not only is affected within its body, but that it also has a sphere of influence exteriorly to the body of the wire, at all points; and advantage is taken of this phenomenon in constructing motors, dynamos, electrical measuring devices and almost every kind of electrical mechanism in existence.

EXTERIOR MAGNETIC INFLUENCE AROUND A WIRE CARRYING A CURRENT.--Bear in mind that the wire coil (A, Fig. 14) does not come into contact with the core (C). It is insulated from the core, either by air or by rubber or other insulating substance, and a current pa.s.sing from A to C under those conditions is a current of _induction_. On the other hand, the current flowing through the wire (A) from end to end is called a _conduction_ current. Remember these terms.

In this connection there is also another thing which you will do well to bear in mind. In Fig. 15 you will notice a core (C) and an insulated wire coil (B) wound around it. The current, through the wire (B), as shown by the darts (D), moves in one direction, and the induced current in the core (C) travels in the opposite direction, as shown by the darts (D).

[Ill.u.s.tration: _Fig. 16._ DIRECTION OF INDUCTION CURRENT]

PARALLEL WIRES.--In like manner, if two wires (A, B, Fig. 16) are parallel with each other, and a current of electricity pa.s.ses along the wire (A) in one direction, the induced current in the wire (B) will move in the opposite direction.

These fundamental principles should be thoroughly understood and mastered.

CHAPTER IV

FRICTIONAL, VOLTAIC OR GALVANIC, AND ELECTRO-MAGNETIC ELECTRICITY

THREE ELECTRICAL SOURCES.--It has been found that there are three kinds of electricity, or, to be more accurate, there are three ways to generate it. These will now be described.

When man first began experimenting, he produced a current by frictional means, and collected the electricity in a bottle or jar. Electricity, so stored, could be drawn from the jar, by attaching thereto suitable connection. This could be effected only in one way, and that was by discharging the entire acc.u.mulation instantaneously. At that time they knew of no means whereby the current could be made to flow from the jar as from a battery or cell.

FRICTIONAL ELECTRICITY.--With a view of explaining the principles involved, we show in Fig. 17 a machine for producing electricity by friction.

[Ill.u.s.tration: _Fig. 17._ FRICTION-ELECTRICITY MACHINE]

This is made up as follows: A represents the base, having thereon a flat member (B), on which is mounted a pair of parallel posts or standards (C, C), which are connected at the top by a cross piece (D). Between these two posts is a gla.s.s disc (E), mounted upon a shaft (F), which pa.s.ses through the posts, this shaft having at one end a crank (G). Two leather collecting surfaces (H, H), which are in contact with the gla.s.s disc (E), are held in position by arms (I, J), the arm (I) being supported by the cross piece (D), and the arm (J) held by the base piece (B). A rod (K), U-shaped in form, pa.s.ses over the structure here thus described, its ends being secured to the base (B). The arms (I, J) are both electrically connected with this rod, or conductor (K), joined to a main conductor (L), which has a terminating k.n.o.b (M). On each side and close to the terminal end of each leather collector (H) is a fork-shaped collector (N). These two collectors are also connected electrically with the conductor (K). When the disc is turned electricity is generated by the leather flaps and acc.u.mulated by the collectors (N), after which it is ready to be discharged at the k.n.o.b (M).

In order to collect the electricity thus generated a vessel called a Leyden jar is used.

LEYDEN JAR.--This is shown in Fig. 18. The jar (A) is of gla.s.s coated exteriorly at its lower end with tinfoil (B), which extends up a little more than halfway from the bottom. This jar has a wooden cover or top (C), provided centrally with a hole (D). The jar is designed to receive within it a tripod and standard (E) of lead. Within this lead standard is fitted a metal rod (F), which projects upwardly through the hole (D), its upper end having thereon a terminal k.n.o.b (G). A sliding cork (H) on the rod (F) serves as a means to close the jar when not in use. When in use this cork is raised so the rod may not come into contact, electrically, with the cover (C).

The jar is half filled with sulphuric acid (I), after which, in order to charge the jar, the k.n.o.b (G) is brought into contact with the k.n.o.b (M) of the friction generator (Fig. 17).

VOLTAIC OR GALVANIC ELECTRICITY.--The second method of generating electricity is by chemical means, so called, because a liquid is used as one of the agents.

[Ill.u.s.tration: _Fig. 18._ LEYDEN JAR]

Galvani, in 1790, made the experiments which led to the generation of electricity by means of liquids and metals. The first battery was called the "crown of cups," shown in Fig. 19, and consisting of a row of gla.s.s cups (A), containing salt water. These cups were electrically connected by means of bent metal strips (B), each strip having at one end a copper plate (C), and at the other end a zinc plate (D). The first plate in the cup at one end is connected with the last plate in the cup at the other end by a conductor (E) to make a complete circuit.

[Ill.u.s.tration: _Fig. 19._ GALVANIC ELECTRICITY. CROWN OF CUPS]

THE CELL AND BATTERY.--From the foregoing it will be seen that within each cup the current flows from the zinc to the copper plates, and exteriorly from the copper to the zinc plates through the conductors (B and E).

A few years afterwards Volta devised what is known as the voltaic pile (Fig. 20).

VOLTAIC PILE--HOW MADE.--This is made of alternate discs of copper and zinc with a piece of cardboard of corresponding size between each zinc and copper plate. The cardboard discs are moistened with acidulated water. The bottom disc of copper has a strip which connects with a cup of acid, and one wire terminal (A) runs therefrom. The upper disc, which is of zinc, is also connected, by a strip, with a cup of acid from which extends the other terminal wire (B).

[Ill.u.s.tration: _Fig. 20._ VOLTAIC ELECTRICITY]

_Plus and Minus Signs._--It will be noted that the positive or copper disc has the plus sign (+) while the zinc disc has the minus (-) sign.

These signs denote the positive and the negative sides of the current.

The liquid in the cells, or in the moistened paper, is called the _electrolyte_ and the plates or discs are called _electrodes_. To define them more clearly, the positive plate is the _anode_, and the negative plate the _cathode_.

The current, upon entering the zinc plate, decomposes the water in the electrolyte, thereby forming oxygen. The hydrogen in the water, which has also been formed by the decomposition, is carried to the copper plate, so that the plate finally is so coated with hydrogen that it is difficult for the current to pa.s.s through. This condition is called "polarization," and to prevent it has been the aim of all inventors. To it also we may attribute the great variety of primary batteries, each having some distinctive claim of merit.

THE COMMON PRIMARY CELL.--The most common form of primary cell contains sulphuric acid, or a sulphuric acid solution, as the electrolyte, with zinc for the _anode_, and carbon, instead of copper, for the _cathode_.

The ends of the zinc and copper plates are called _terminals_, and while the zinc is the anode or positive element, its _terminal_ is designated as the positive pole. In like manner, the carbon is the negative element or cathode, and its terminal is designated as negative pole.

Fig. 21 will show the relative arrangement of the parts. It is customary to term that end or element from which the current flows as positive. A cell is regarded as a whole, and as the current pa.s.ses out of the cell from the copper element, the copper terminal becomes positive.

[Ill.u.s.tration: _Fig. 21._ PRIMARY BATTERY]

BATTERY RESISTANCE, ELECTROLYTE AND CURRENT.--The following should be carefully memorized:

A cell has reference to a single vessel. When two or more cells are coupled together they form a _battery_.

_Resistance_ is opposition to the movement of the current. If it is offered by the electrolyte, it is designated "Internal Resistance." If, on the other hand, the opposition takes place, for instance, through the wire, it is then called "External Resistance."

The electrolyte must be either acid, or alkaline, or saline, and the electrodes must be of dissimilar metals, so the electrolyte will attack one of them.

The current is measured in amperes, and the force with which it is caused to flow is measured in volts. In practice the word "current" is used to designate ampere flow; and electromotive force, or E. M. F., is used instead of voltage.

ELECTRO-MAGNETIC ELECTRICITY.--The third method of generating electricity is by electro-magnets. The value and use of induction will now be seen, and you will be enabled to utilize the lesson concerning magnetic action referred to in the previous chapter.

MAGNETIC RADIATION.--You will remember that every piece of metal which is within the path of an electric current has a s.p.a.ce all about its surface from end to end which is electrified. This electrified field extends out a certain distance from the metal, and is supposed to maintain a movement around it. If, now, another piece of metal is brought within range of this electric or magnetic zone and moved across it, so as to cut through this field, a current will be generated thereby, or rather added to the current already exerted, so that if we start with a feeble current, it can be increased by rapidly "cutting the lines of force," as it is called.

DIFFERENT KINDS OF DYNAMO.--While there are many kinds of dynamo, they all, without exception, are constructed in accordance with this principle. There are also many varieties of current. For instance, a dynamo may be made to produce a high voltage and a low amperage; another with high amperage and low voltage; another which gives a direct current for lighting, heating, power, and electroplating; still another which generates an alternating current for high tension power, or transmission, arc-lighting, etc., all of which will be explained hereafter.

In this place, however, a full description of a direct-current dynamo will explain the principle involved in all dynamos--that to generate a current of electricity makes it necessary for us to move a field of force, like an armature, rapidly and continuously through another field of force, like a magnetic field.

DIRECT-CURRENT DYNAMO.--We shall now make the simplest form of dynamo, using for this purpose a pair of permanent magnets.

[Ill.u.s.tration: _Fig. 22._ DYNAMO FIELD AND POLE PIECE]

SIMPLE MAGNET CONSTRUCTION.--A simple way to make a pair of magnets for this purpose is shown in Fig. 22. A piece of round 3/4-inch steel core (A), 5-1/2 inches long, is threaded at both ends to receive at one end a nut (B), which is screwed on a sufficient distance so that the end of the core (A) projects a half inch beyond the nut. The other end of the steel core has a pole piece of iron (C) 2" 2" 4", with a hole midway between the ends, threaded entirely through, and provided along one side with a concave channel, within which the armature is to turn.