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ments, arise to some extent from the irrational basis on which the British Association system of units has been built up. It was felt therefore essential to give the reader a brief account of the rational system of units proposed by Mr. Oliver Heaviside. The general facts of the subject have, however, been explained and stated in the text in terms of the usual definitions and language, because in a short elementary treatise intended for the use of beginners, it is not desirable to depart too far from the use of expressions which are current in more advanced text-books, and to which such a volume as the present one is intended to be introductory. The Author has also followed the ordinary custom in using such terms as electromotive force, magnetomotive force, and magnetic force. There can be but little doubt, however, that students would sooner acquire clear notions on the subject of quantitative electromagnetism, if the multiplicity of equivalent terms, and especially the customary extended use of the word force, were discontinued by common consent. A clear and unambiguous magnetic phraseology could be constructed on the following basis:

Let the idea now conveyed by the term magnetomotive force be expressed, as Mr. Heaviside and other writers have already done, by the word gaussage (pronounce “gowsage”), and let the gaussage between any two points or around any magnetic circuits be measured in a unit termed a gauss, such that 80 ampere-turns are equal to one microgauss.

Let electromotive force similarly be called as usual, voltage, and measured in volts.

Let the gaussage per centimetre along any direction at any place, be called the gaussivity at that point Gaussivity is therefore the name for that quantity which is commonly called the magnetic force or the magnetic intensity at any point.

Similarly, let voltivity be the name for the voltage per centimetre in any direction, or for the so-called electric force at any point.

The term magnetic flux, or simply the flux or fluxage, may then very well be used to denote what is commonly called the total induction, and be stated to be measured in units called webers.

The flux density might then consistently be called the fluxivity at any point.

The essential qualities of a ferromagnetic body are then defined, as explained in the text, by the terms retentivity, coercivity, reluctivity, and permeability, and the qualities of a magnetic circuit, by the terms, reluctance and permeance, as usually is the case.

The fluxivity is therefore the name for the magnetic state produced by gaussivity at any point in empty or matter-occupied space.

A distribution of gaussivity exists in the regions near all electric currents or magnetic poles.

Retentivity is the property possessed by ferromagnetic bodies, of maintaining fluxivity when the impressed gaussivity is withdrawn.

The reluctivity of the material is the measure of the gaussivity required to produce unit fluxivity at any point. The energy stored up per unit of volume of the magnetic circuit is measured by half the product of the gaussivity and fluxivity at that point. By the use of these or similar terms the student is led to recognise clearly the relations of the magnetic measurable quantities. He is not confused by the employment of the term force in other than its strict dynamical sense, and this latter word is reserved entirely as the name for the cause, whatever its nature, which effects a change in momentum in material substances. The analogies as far as they exist, between mechanical stress and strain and magnetic gaussivity and fluxivity are easily seen and pointed out. If similarly the total current through any area is called the amperage (pronounce "ampeerage"), and the rational system of units adopted, at least in theory, we should have a simple and easily remembered series of statements concerning the fundamental relations and methods of measuring the various magnetic and electric quantities concerned in electromagnetism. The two principal relations would then admit of the simplest possible statement as follows.

In a co-linked electric and magnetic circuit, such as exists in a dynamo or transformer, the gaussage round the magnetic circuit is numerically equal to the amperage through the magnetic circuit. Any change in the fluxage through the electric circuit produces a voltage round the electric circuit, numerically equal to the time rate of change of the fluxage.

The methods of marking out spaces occupied by magnetic flux by "lines" and "tubes” are useful from a mathematical point of view, but the practical student learns more readily to submit the facts to arithmetical calculation when he discards these notions, and thinks of magnetic flux or fluxage merely as a physical state existing in a magnetic circuit which is measured in webers or microwebers, just as he thinks of electric current or amperage as another kind of physical state in an electric conducting circuit, and which is measured in amp:res or milliamperes, as the case may be. In the first case the active cause producing flux or fluxage is called gaussage, and in the second case the active cause of current or amperage is called voltage. Electric and magnetic terminology is undoubtedly in a transition condition. Terms and phrases based upon “action-ata-distance” notions and "fluid” theories still survive, and are intermixed with others of later coinage.

The objection which is generally raised to the introduction of new words and terms may be met by the contention that whilst scientific progress both in research and teaching is assisted up to a certain stage by the use of language which involves some hypothesis, or by words descriptive of some imagined state or process, yet beyond that point it is desirable to divest terminology of unnecessary hypotheses, and that therefore new terms are needed to replace the older ones when these last are found to suggest erroncous ideas, or to involve undesirable ambiguity of language.

In conclusion, the Author desires to express his thanks for the loan of blocks for some diagrams to the Society of Arts, the Electrician Publishing Company, Messrs. Elliott Bros., Mr. James Pitkin, and the General Electric Company, and also in particular to Mr. W.C. Clinton for kind assistance in revising the proof sheets.

J. A. F.

UNIVERSITY COLLEGE, LONDON:

December 1897.

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