72 -nature's 



The field may revolve or the electromagnet may 

 revolve, whichever is the most convenient to 

 construct. The field-magnet may be a per- 

 manent magnet or an electromagnet, made per- 

 manent during the operation of the dynamo by 

 a part of the current generated by the machine 

 being directed through a coil surrounding soft 

 iron; or the field-current may come from an 

 outside source. This is the kind of field-mag- 

 net universally used for dynamo work, as a 

 much stronger magnetism is developed in this 

 wa> than it is possible to obtain from any sys- 

 tem of permanent steel magnets. 



The usual construction is to have a station- 

 ary field-magnet and then a series of electro- 

 magnets mounted and revolving upon a shaft 

 in the center of the magnetic field. The rotat- 

 ing part is called the armature, and is so 

 wound with insulated wire that successive in- 

 duced currents are created in the armature 

 windings and discharged through brushes 

 which rest on revolving segments that connect 

 with the armature wi in I ings. These induced 

 currents succeed each other with such rapidity 

 as to amount in practice to a steady current. 

 However, the separate pulsations are easily 

 heard in any telephone when the circuit is 

 near to that of a dynamo circuit. The dy- 

 namo current is not nearly so steady as the 

 battery current, although both are probably 

 made up of separate discharges. In the dy- 



