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freedom from danger. The underwriters have done much to bring about the improved state of affairs, for surety of action and freedom from fire causation are very closely related.
Faulty circuits are not always easily discovered, for there is often no sensible evidence of defects until they are looked for with great painstaking or until they suddenly become worse and give sensational notice of fault. It is consequently of great importance that the work be done thoroughly at the start. It is not a difficult matter to confine electricity to its proper conductor, but to do so it is absolutely necessary that every care be taken with the insulation. With all metals such good conductors, and with brickwork, plaster, and damp wood conductors in some degree, there is great liability that the current will go astray.
It is common to compare a current of water with a current of electricity, but there is one striking difference. Water flows freely in space, and to conduct it we surround a certain amount of space with metal — we use a pipe. The metal is the insulator through which the water cannot pass unless the metal be punctured. Electricity does not "flow" freely in space, but does through metals. Space is the impassable and the metal is the "hole." When wires are strung it may be looked upon as punching holes for the electricity to flow in, and if these "holes" are not surrounded by some non-conductor it is of course impossible to keep the electricity in the path intended for it. Pipes are strengthened and joints are looked after with regard to the pressure upon them. Electrical insulation should also be looked after with regard to the pressure which it is to withstand, for there is an electro-motive force that is analogous to water-pressure. In insulating wires the difficulty comes from the fact that electric insulators can be either punctured or permeated by conductors. Sometimes the insulators themselves become conductors through the action of heat or other deteriorating agents, and it is thus necessary to provide, in one way or another, against a number of accidents. If a nail pierce the insulation, if dampness permeate it, or if it become charred by heat, there may be leakage. It is as if a water-system were tapped by small pipes through which the water constantly leaked away, only the waste with electricity is far more subtle and attracts no attention unless, because of the leak, there is a lack of power in the circuit. And it is by no means the waste only that makes leaks undesirable. There is often a very small margin between a small leak and a great one, and if it is large it carries off practically all of the electricity, leaving nothing for useful purposes, and in doing so is very apt to cause a part of the circuit to heat to a dangerously high temperature. But even in damp cellars, tunnels, and mines, electric circuits are in successful operation. If a few principles are kept in mind and a few rules carefully followed leaks will be impossible under normal conditions.
The strangeness of electrical terms is, perhaps, largely accountable for the lack of knowledge about electrical matters. It is discouraging to meet at the start with "volts," "amperes," "ohms," "watts," and other names and expressions as mysterious as the cabalistic signs on a physician's prescription. But it is only another illustration for Carlyle's "Philosophy of Clothes." Electricity is not the only subject that is looked upon with increased respect and awe because of the peculiar names given to ideas concerning it.
This clothing of the ideas no doubt obscures many simple facts and fails to suggest their semblance to ideas that are familiar, but in the end it is an aid to clearness to have the terms distinctive. When once there is clear conception of the fundamental ideas, the electrical nomenclature is found simple and direct, and it is an advantage to have specific terms that will not suggest too readily, foreign ideas not analogous in all particulars. It is, perhaps, this very directness, the absence of all roundabout terms, that makes most electrical talk sound highly technical. It is encouraging to the uninformed to meet in explanations with "pounds," "feet," and other common quantities, because they are familiar, and the most abstruse talk in some way seems to gain clearness when this much is understood. It is, however, largely a question of familiarity, and it is doubtful that our knowledge of the ultimate meaning of pounds and feet is much clearer than our knowledge of volts and amperes.
Analogies are sometimes confusing because they are taken too literally, and the attempt to reconcile the analogous ideas in all particulars will sometimes cause more confusion than if a more abstract conception were attempted at the start. Thus, although it is often helpful, it is not necessary to compare electrical forces and quantities with those that seem to have a more material existence. Instead of comparing them with pressure of steam, currents of water, or resistance of friction, we may compare them, for instance, with the "pressure of public opinion," "currents of feeling," "resistance to changes." These expressions have a definite meaning to us even though the ideas themselves do not appeal to the five material senses. If there were a way to measure "the pressure of public opinion," we should probably do so, fix upon a unit and give a name to it; but when we cannot do this and yet can measure "electrical pressure," it would seem as if the latter should be not less clearly recog nized.
We have these abstract ideas of pressure, resistance, and current, and if they are kept in mind while the names are looked upon as figures of speech rather than as literal definitions, the ideas are of great usefulness.
There is what is termed "electrical pressure" or electro-motive force. It is a definite force and can be measured, although it does not show its effect in quite the same way that water-pressure or steam-pressure does. We do not see it push masses of substance before it, but the abstract idea of pressure can be applied to it just as we apply the idea to "pressure of public opinion." It causes certain actions in an electric circuit. When the force is great the action is great, when it is small the action is accordingly small. It agrees with our idea of pressure, and by thinking of it as such, a clear conception of its nature is helped. Resistance, current, and other electrical quantities may be looked at in the same way. It is not necessary to assume that there is an actual flow of something material. It is not known that anything material actually passes through a wire, but it is known that there is a progressive effect along it as if there were a "current" of something, and that this effect is produced by the electro-motive force which acts "like a pressure."
When once it became known that definite electrical forces and quantities exist and give rise to certain effects, and when these had been named, it remained of course to decide upon units, so that these forces and quantities might be measured and compared. It is