CHAPTER I

THE MAGNET AND THE MAGNETIC FIELD

A.-The simple natural magnet·

1. The lodestone. It was known to the ancients that certain ores of iron possess the property of attracting and holding to them, in a peculiar way, small particles of iron. Most notable in this respect was an ore from a mine in the province of Magnesia, in Asia Minor, so that the term 'magnetic stone' ( Mayvñτis Xilos) came to have a special significance, the name 'magnetic' being applied to the entire group of phenomena with which we are now concerned.

The stone itself is shortly called a magnet, or, to distinguish it from those which are artificially produced, a 'natural magnet.'

The lodestone, or magnetic iron ore, is a combination of equivalent parts of ferrous oxide and ferric oxide, and has the formula Fe 0, Fe2 03. It is found for the most part in dense homogeneous pieces of a lustrous black colour, and when crystallised has the form of the regular system. Next to the lodestone in magnetic properties is the magnetic sulphide of iron (Fe S, Fe2 S3). The same properties are found to exist to a much smaller extent in the ores of nickel and cobalt.

2. The magnetic property.-Experiment 1.-Let a piece of lodestone be dipped in iron filings: tufts and strings of filings remain hanging to the lodestone in many places, though not in all, more especially at projecting corners and edges.

If any other body, such as a finger, be dipped into the filings, it may be that some few particles will remain adhering to it, but

the adhesion is evidently of an entirely different kind. The comparison shows that the lodestone exerts some special influence on the iron filings.

If the experiment be repeated on substances other than iron filings, the lodestone being dipped, for example, into particles of copper or brass, or into silica (Si O2) in the form of sand, we shall only notice that kind of cohesion which exists between all bodies in contact, and not any special action of the lodestone on the particles.

It is not to be concluded from this that the bodies in question are never susceptible to magnetic influence. We shall find later, when we have learnt how to produce much stronger magnetic effects, that all substances, even gases, respond in some measure to magnetic influences. Our experiment only shows, then, that iron responds in a far more notable degree. We shall accordingly make use of iron for the closer investigation of magnetic phenomena. We speak of it as 'strongly magnetic' in distinction from 'feebly magnetic' bodies. There are no unmagnetic bodies.

3. Conception of a magnet.-Any body which exerts the same influences as the lodestone we shall call a 'magnet.'

Accordingly we should speak of the so-called 'galvanic current' as a magnet, because in treating of it we start from a consideration of its magnetic influence, which we make directly evident by means of iron filings, just as in the present section we do with all ordinary magnets.

4. Closer investigation of the nature of magnetic effects. Experiment 2.-On examining a piece of lodestone which has been dipped in iron filings, the adherent tufts appear not only to be subject to the influence of gravity, but to be themselves the seat of some special kind of influence. Between the separate particles some modifying cause is evidently at work; they possess a certain rigidity. If we try to separate them from the lodestone they become drawn out into chains, the filings adhering one to another in succession.

This is seen especially clearly when the filings are sup

ported from below, so that they are only in part dependent from the magnet.

Experiment 3.-Scatter some iron filings over a piece of white paper, and over the sheet draw a corner of the lodestone which was found in experiment 1 to be especially active : a string of filings forms and grows in length, the particles already attached acquiring the property of the lodestone, and attracting others.

The magnetic influence is thus taken up by the chains of particles, and transmitted from one particle to another.

The effect is greatest near to the lodestone, and diminishes at greater distances; when the chain is too long it breaks off, the successively diminishing magnetic forces being finally insufficient to overcome the frictional forces conditioned by the weights of the separate particles.

5. Diagrams of lines of force.-The method of procedure so far employed, of dipping a lodestone into iron filings, and examining the manner in which the influence is taken up and transmitted from particle to particle, is, however, imperfect. We must further take care that, in the building up of the chains of influence which we wish to study, the effect of gravity shall be as far as possible eliminated. This result can be most easily attained by scattering the filings over a horizontal supporting surface, which passes through a region of magnetic influence.

Experiment 4.-By means of a sieve, let iron filings be scattered from a greater height, so that they may be distributed as uniformly as possible over the horizontal sheet of paper. Next let the lodestone, freed from any filings that may already be adhering to it, be laid upon the paper, which is then tapped in various places. The filings arrange themselves in regularly curved lines (fig. 1); these are the socalled Faraday's lines of force.

These iron-filing pictures were known before Faraday's time; but their true significance was not appreciated. Faraday made them the starting point of a series of important and fundamental conceptions, and accordingly we name them after him.

These lines of force show very clearly how the influence exerted by the lodestone and transmitted from filing to filing is in a certain sense conducted.' It should be observed that, even at points somewhat distant from the lodestone, the filings take up a definite direction, corresponding to the

course of the lines of force which pass through their positions.

6. Practical hints on the production of pictures of magnetic lines of force.-As we shall often avail ourselves in the following pages of the very important aid of these line-of