5o8 THE POPULAR SCIENCE MONTHLY. 



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 arma- 

 ture wire joined to itself then a mysterious and extraordinary 

 resistance to motion at once occurs. If we are turning the arma- 

 ture 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 mechani- 

 cal friction, and nothing directly corresponding to the ordinary 

 losses of power in other machines. 



This wonderful result has been analyzed into three funda- 

 mental 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 non- 

 magnetic matter, like copper or India rubber. 



Second. As soon as the wire moves so as cut through these in- 

 visible 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 electro- 

 motive force is caused in some way by motion in a magnetic field. 



