Page:Popular Science Monthly Volume 46.djvu/524

508 saying it is alive. If we can not strictly and literally call it so, yet there are such broad features in common that I think we may study it as one of the nearest approximations by mechanism to some of the simplest forms of actually living creatures.

The general arrangement of the apparatus is as follows: A number of magnets, with their ends, or poles, alternately north and south, are arranged around a circle with the magnet legs pointing inward toward the center, but not reaching it. Within the smaller circle thus formed a few loops of copper wire wound on an iron drum revolve, but without touching the magnets or outer frame. So long as the inwardly projecting legs are not magnetized the armature, as the coils of wire are called, revolves freely, and no effort on the part of the engine or other source of power is required to turn it except sufficient to overcome the slight mechanical friction of the shaft. Also, if the magnets are excited and the copper wire of the armature does not have its ends joined so as to form a complete return path, there is no opposition to the rotation. But when both of these conditions are supplied—viz., the magnets, also called the "field," are excited and the armature wire joined to itself—then a mysterious and extraordinary resistance to motion at once occurs. If we are turning the armature by hand, it feels as though we were forcing it through thick jelly. If more force, such as that of a steam engine, is applied, it may take many horse power to revolve the armature rapidly, and yet there is no scraping or contact between the surfaces of the armature and field, nothing giving rise to ordinary mechanical friction, and nothing directly corresponding to the ordinary losses of power in other machines.

This wonderful result has been analyzed into three fundamental conditions, often called causes. They are mysterious, like the original phenomenon; but, then, every appearance in Nature is a mystery to the last analysis. These three are, first, a peculiar force emanating from the ends of the field magnets and extending from pole to pole by curved paths, called "lines of force," going through space, whether filled with substance or entirely empty. They are probably lines of stress in the ether, and we know that any metallic body placed in the path of these lines is submitted to the influence of the lines of force. Whatever this influence is, it does not give rise to anything perceptible to our senses in nonmagnetic matter, like copper or India rubber.

Second. As soon as the wire moves so as cut through these invisible lines of force, a new stress, called electro-motive force, is produced in it, and now the free ends of the copper wire have suddenly acquired the property of attracting each other, but the magnitude of this attraction is exceedingly small. This electromotive force is caused in some way by motion in a magnetic field.