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An excerpt from the beginning of the INTRODUCTION:
The study of electricity and magnetism as represented in the following chapters is independent of any consideration of the nature of the physical action which leads to the production of electromotive force in a voltaic cell or dynamo; it is independent of any consideration of the nature of the physical action which constitutes an electric current in a wire; it is independent of any consideration of the nature of the disturbance which constitutes a magnetic field; and it is independent of any consideration of the nature of the disturbance or stress which constitutes an electric field. This kind of study of electricity and magnetism may very properly be called electro-mechanics.
Simple mechanics is the study of ordinary bodies at rest or in visible motion, and one of the most important ideas in mechanics is the idea of force, but the science of mechanics is not concerned with, and indeed it sheds no light upon, the question as to the physical nature of force. Thus, the science of mechanics is not concerned with the question as to the nature of the action which takes place in a gas and causes the gas to exert a force on a piston; the science of mechanics is not concerned with the question as to the nature of the action which takes place in the material of a stretched wire causing the wire to exert a pull upon each of the two supports at its ends; the science of mechanics is not concerned with the nature of the action between the earth and a heavy weight which causes the earth to exert a force on the weight. It is sufficient for the science of mechanics that these things are what may be called states of permanency which involve certain invariant co-relations. Thus, in the case of a stretched wire there is a certain invariant relation between what we call the value of the stretching force and the amount of elongation, In the case of a gas there is a certain invariant relation between the density of the gas and its pressure, and so on.
Similarly it is sufficient for the science of electro-mechanics that such things as electric current, electromotive force, magnetic field and electric field are states of permanency which involve invariant co-relations.
The character of the science of mechanics and of the science of electro-mechanics may be further exemplified as follows: A sample of steel under test is broken by a tension of 120,000 pounds, but the exact character of the action which takes place in the steel when it is placed under tension is not a matter for consideration. Neither does one need to consider the action which takes place in the furnace of the boiler which supplies steam to the engine which drives the dynamo which supplies current to the motor which drives the testing machine! A plate of glass under test is broken down and punctured by an electromotive force of 95,000 volts, but the exact character of the action which takes place in the glass when it is subjected to the electro-motive force is not a matter for consideration. Mechanics is concerned with the correlation of what may be called lump effects, such as the relationship between the size of a beam and the load it can carry, the size of a fly wheel and the work it can do when stopped, the thickness and diameter of a boiler shell and the pressure it can stand, the size of a submerged body and the buoyant force which acts upon it, the size and shape of the air column in an organ pipe and its number of vibrations per second, the thickness of a glass plate and the electromotive force it can stand, the size of a copper wire and the current it can carry with a given rise of temperature and so forth.
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