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THE POPULAR SCIENCE MONTHLY. 



which are wound a number of insulated coils. Radial pieces connected 

 with the end of the wire of one coil and the beginning of the next con- 

 duct the currents to the commutator cylinder, when they are taken off 

 by brushes. The armature is mounted upon a shaft and revolves be- 

 tween the poles of an electro-magnet, the arrangement being that 

 shown in Fig. 16. When the machine is used on an electro-motor, the 

 current enters the armature by the brushes and is alternately changed 

 in direction by the commutator, so that there is attraction as the arma- 

 ture approaches and repulsion as it leaves the poles of the field magnet. 



Fig. 16. 



Besides machines constructed primarily to generate currents, but 

 which may be used as electro-motors as well, there are various others 

 designed especially for the latter purpose. The commonest form of 

 these is a set of electro-magnets arranged radially around the arma- 

 ture, which is rotated under the influence of the changing polarity, or 

 the magnetization and demagnetization of the field-magnets. 



One of the best of the machines constructed for motor purposes, 

 and one that has received high praise from competent electricians, 

 is that of M. Marcel Deprez. It consists of a compound permanent 

 horseshoe magnet with a Siemens armature between its poles. This 

 latter is simply a soft-iron cylinder with two longitudinal grooves in 

 which insulated copper wire is wound, the poles being the faces of 

 the cylinder between the coils. It is placed with its axis parallel to 

 the arms of the magnet instead of across them as is ordinarily done, and 

 to this arrangement is due its greater power, as the whole strength of 



