Ans. The strongest magnetism resides in the ends, while all around the magnet half way between the poles there is no attraction at all.

[Ill.u.s.tration: FIG. 99.--Magnetic poles. If a bar magnet be plunged into iron filings and then lifted, as ill.u.s.trated in the figure, a ma.s.s of filings will cling to the ends of the magnet but not to the middle. The ends are called the _poles_ of the magnet.]

=Ques. How are the poles designated?=

Ans. They are called the _north pole_ and the _south pole_.

=Ques. What is the distinguishing feature of each?=

Ans. The north pole points approximately to the earth's geographical north, while the south pole of a magnet points approximately to the earth's geographical south.

The north pole is the positive (+) pole and the south pole is the negative. The north and south poles were formerly called in France, the austral and boreal poles respectively.

=Magnetic Field.=--When a straight bar magnet is held under a piece of card board upon which iron filings are sprinkled, the filings will arrange themselves in curved lines radiating from the poles. If a horse shoe magnet be held at right angles to the plane of the card board, the filings will arrange themselves in curved lines, as shown in fig. 108. These lines are called _magnetic lines of force_ or simply _lines of force_; they show that the medium surrounding a magnet is in a state of stress, the s.p.a.ce so affected being called the _magnetic field_.

[Ill.u.s.tration: FIG. 100.--Badly magnetized bar. Properly magnetized magnets have only two poles; it is possible, however, by special or careless magnetization, to produce magnets with more than two poles, but no process will produce a magnet with a single pole. If an abnormal magnet with more than two poles be dipped into iron filings, the latter will adhere at places other than the two ends, as shown in the ill.u.s.tration.

The polarities are alternately N and S; that is, the regions N, B, N, have north polarity, while A and C have south polarity. These are known as _consequent poles_.]

[Ill.u.s.tration: FIGS. 101 to 107.--Effect of breaking a magnet into several parts. If a magnetized needle be broken, each part will be found to be a complete magnet having a N and S pole. The sub-division may be continued indefinitely, but always with the same result as indicated in the figure.

This is evidence of the correctness of the molecular theory of magnetism, which states that _the molecules of a magnet are themselves minute magnets arranged in rows with their opposite poles in contact_.]

=Ques. What is the extent and character of the magnetic field?=

Ans. The influence of a magnet is supposed to extend in all directions indefinitely, however, the effect is very slight beyond a comparatively limited area.

[Ill.u.s.tration: FIG. 108.--The region about a magnet in which its magnetic forces can be detected is called the _magnetic field_. This can be represented graphically by placing a piece of cardboard over the magnet and sprinkling iron filings on the paper, gently tapping at the same time.

Each filing becomes a temporary magnet by _induction_, and sets itself, like the compa.s.s needle, in the direction of the _line of force_ of the magnetic field.]

[Ill.u.s.tration: FIG. 109.--Tracing lines of force with a suspended magnet.

If a small magnetic needle, suspended by a thread, be held near a magnet, it will point in some fixed direction depending on the proximity of the poles of the magnet. The direction taken by the magnet is called the direction of the force at the point, and if the suspended needle be moved forward in the direction of the pole, it will trace out a curved line which will be found to start from one of the poles, and end at the other.

Any number of such lines can be traced; the s.p.a.ce filled by these lines of force is called the _magnetic field_.]

[Ill.u.s.tration: FIG. 110.--Magnetic action: _Unlike poles of magnets attract each other_.]

[Ill.u.s.tration: FIG. 111.--Magnetic action: _Like poles of magnets repel each other_.]

=Magnetic Force.=--This is the force with which a magnet attracts or repels another magnet or any piece of iron or steel. The force varies with the distance, being greater when the magnet is nearer and less when the magnet is farther off. The following are the laws relating to magnetic force:

1. _Like magnetic poles repel one another; unlike magnetic poles attract one another._

2. _The force exerted between two magnetic poles varies inversely as the square of the distance between them._

=Magnetic Circuit.=--The path taken by magnetic lines of force is called a magnetic circuit; the greater part of such a circuit is usually in magnetic material, but there are often one or more air gaps included. The total number of lines of force in the circuit is known as the _magnetic flux_.

=Ques. How is magnetic flux measured?=

Ans. By a unit called the maxwell.

Named after James Clerk Maxwell the Scottish physicist.

=Ques. What is the maxwell?=

Ans. _The amount of magnetism pa.s.sing through every square centimetre of a field of unit density._

=Ques. What is the unit of field strength?=

Ans. The gauss.

=Ques. What is a gauss?=

Ans. _The intensity of field which acts on a unit pole with a force of one dyne. It is equal to one line of force per square centimetre_. Named after Karl Friedrich Gauss, the German mathematician.

=The Magnetic Effect of the Current.=--Hans Christian Oerstead, the Danish scientist, discovered in 1819 that a magnet tends to set itself at right angles to a wire carrying an electric current. He also found that the way in which the needle turns, whether to the right or left of its usual position, depends: 1, upon the position of the wire that carries the current, whether it be above or below the needle, and 2, on the direction in which the current flows through the wire.

[Ill.u.s.tration: FIG. 112.--Ill.u.s.trating Maxwell's "corkscrew rule" for relative directions of current and lines of force. According to the rule: _the direction of the current and that of the resulting magnetic force are in the same relation to each other as is the forward travel and rotation of an ordinary corkscrew_, Thus, in the figure, if a current flow through the wire _ab_ in the direction from _a_ to _b_, the magnetic lines will encircle the wire in the direction of the curved arrow _ro_ which shows the direction in which the corkscrew must be turned to advance in the direction of the arrow _n_.]

To keep these movements in mind numerous rules have been suggested, of which the following will be found convenient:

=Corkscrew Rule=.--_If the direction of travel of a right handed corkscrew represent the direction of the current in a straight conductor, the direction of rotation of the corkscrew will represent the direction of the magnetic lines of force._

[Ill.u.s.tration: FIG. 113.--Experiment showing direction of lines of force in the magnetic field surrounding a conductor carrying an electric current. A piece of copper wire is pierced through the center of a sheet of cardboard, and carried vertically for two or three feet then bent around to the terminals of a battery or other source of current. If iron filings be sprinkled over the card while the current is pa.s.sing, they will arrange themselves in circles around the wire, thus indicating the form of the magnetic field surrounding the conductor. Compa.s.s needles may also be used to show the direction of the lines of force at any point.]

[Ill.u.s.tration: FIG. 114.--Right hand rule to determine the direction of magnetic field around a conductor carrying a current. The thumb of the right hand is placed along the conductor, pointing in the direction in which the current is flowing, then, if the fingers be partly closed, as shown in the ill.u.s.tration, the finger tips will point in the direction of the magnetic whirls.]

=Ques. What is the effect of a current flowing in a loop of wire?=

Ans. If, in figs. 116 and 117, the current flow in the direction indicated by the arrow, the lines for magnetic force are found to surround the loop as shown; all the lines leave on one side of the loop and return on the other; accordingly, a north pole is formed on one side, and a south pole on the other.

[Ill.u.s.tration: FIG. 115.--Right hand palm rule to determine the direction of the magnetic field around a conductor carrying a current: Place the palm of the outstretched right hand above and to the right side of the wire with the fingers pointing in the direction of the current, and the thumb extended at right angles, that is, pointing downward. The direction in which the thumb points will indicate the direction of the magnetic whirls.]

[Ill.u.s.tration: FIG. 116.--Lines of force of a circular loop. If a current flow through the loop in the direction indicated, the lines of force both inside and outside the loop, will cross the plane of the loop at right angles, and all those which cross the loop on the inside will pa.s.s through the plane in one direction (downwards in the figure), while all on the outside will return through the plane in the opposite direction.]

=Solenoids.=--A solenoid consists of a spiral of conducting wire wound cylindrically so that, when an electric current pa.s.ses through it, its turns are nearly equivalent to a succession of parallel circular circuits, and it acquires magnetic properties similar to those of a bar magnet.

[Ill.u.s.tration: FIG. 117.--Lines of force of a circular loop. If the loop pa.s.s through a piece of cardboard at right angles to its plane, and the current flow as indicated, the dotted lines on the cardboard will represent the direction of the lines of force in the plane of the cardboard. The student should verify the lines of force as here given by applying the corkscrew rule.]

=Ques. What is the character of the lines of force of a solenoid in which a current is flowing?=

Ans. The lines of force must be thought of as closed loops linked with the current. The conductor conveying the current pa.s.ses through all the loops of force, and these are, so to speak, threaded or slung on the current-line of flow, as in fig. 116.

=Ques. What is the distribution of the lines of force?=

Ans. The lines of force form continuous closed curves running through the interior of the coil; they issue from one end and enter into the other end of the coil, as shown in fig. 117.

=Ques. What are the properties of a solenoid?=

Ans. A solenoid has north and south poles, and in fact possesses all the properties of an ordinary permanent magnet, with the important difference that the magnetism is entirely under control.