joYnaMo-electric machines. 89 



magnetic effect upon that layer is only one-fourth what it would be if 

 it was possible to run it without clearance. That the effect on the 

 second layer is in the ratio of nine to four, on the third layer, sixteen 

 to four, and on the fourth layer, twenty to four. The amount of current 

 generated in each layer, twenty is proportional to the magnetic effect. 

 The inner layers or those nearest the armature, have thus a 

 retarding effect on the current generated in the outer ones. This 

 could be carried to the extent of one being entirely neutralized 

 by the other, so that the whole of the current generated in a 

 machine might be spent in producing heat only, in the conductor 

 covering the armature. The iron forming the interior of the arm- 

 ature, that is, the portion next to the wire conductor, comes to the 

 assistance of the inner layers, because, as it is magnetized by induc- 

 tion, with opposite polarity to the field magnet, it in turn helps the 

 generation of currents in those turns of wire next it, but not so strongly 

 as the outer layers, because of its magnetism being feebler, as in 

 no mechanical action can the part acted upon be as strongly afftcted 

 as the part which performs the action. This action and reaction con- 

 tinues so long as the armature is in motion. The operation is 

 continued, but in a contrary sense, when the armature passes from 

 one pole to another. The quantity of iron that may be contained in 

 the armature is limited, because the larger the mass, the greater the 

 time required to magnetize and demagnetize it. Not only is the 

 quantity limited, but it is better to make it up of a number of turns 

 of small iron wire, or a number of thin sheets of iron side by side. 

 It has been found that small bodies of iron receive magnetism and 

 part with it very readily; much more so than large masses. This readi- 

 ness to change its polarity, on the part of an armature composed of 

 wire, or thin sheets of iron, possesses two very great advantages, which 

 are sometimes lost sight of. One is, that, as it absorbs magnetism 

 very readily, it consequently absorbs it to a greater degree, and the 

 other is, that as it both absorbs and parts with its magnetism readily, 

 it becomes less heated than it otherwise would do. These properties 

 enable the machine to be run much faster than it could be otherwise. 

 And as the work done by a machine is greater than the ratio of the 

 square of its speed, you see how advantageous it is to construct 

 machines that can be run, not only at a high rate of speed, but which 

 will absorb as little power as possible. 



