2 DIRECT CURRENT GENERATORS. [Exp. 



direct current generators and motors with a stationary field and 

 revolving armature. 



2. The student should consult any of many excellent treatises 

 for a detailed discussion of different types of generators and if 

 possible should note one or two machines in the laboratory or 

 elsewhere which are examples of each important type. Machines 

 noted should illustrate the following terms, some of which are 

 briefly explained in later paragraphs : Stationary field, revolving 

 field, bipolar, multipolar, separately-excited, self-excited, series 

 wound, shunt wound, compound wound, magneto-generator, open 

 and closed coil armature, drum armature, Gramme (or ring) 

 armature. 



Only the general structure of the various machines need be 

 noted. Observe particularly the magnetic circuit of each machine 

 and the disposition of the field winding. Keep in mind that 

 magnetic flux is proportional to magnetomotive force (field 

 ampere-turns) divided by the reluctance of the complete magnetic 

 circuit, i. e., the sum of the reluctance of each part (air gap, core, 

 etc.). As the reluctance of any part of a magnetic circuit is equal 

 to the length divided by the product of the cross-section and per- 

 meability, it is obvious that an unnecessarily long magnetic circuit 

 should be avoided, a fact neglected in some early machines. 



3. In a bipolar generator, one pole is north and the other 

 south; in a multipolar generator (with 4, 6, 8, etc., poles) the 

 poles are alternately north and south. 



Each armature conductor accordingly passes first underneath a 

 north and then underneath a south pole and has induced in it 

 an electromotive force first in one direction* and then in the 



*(3a). An exception is the so-called unipolar, homopolar or acyclic 

 dynamo, which has a unidirectional electromotive force generated in the 

 armature conductor; it accordingly delivers direct current to the line with- 

 out commutation. Faraday's disk dynamo (one of the earliest dynamos) 

 was of this type. For years it was the dream of zealous electricians to 

 make this type of machine practicable, but it was considered only as an in- 

 teresting freak, for at ordinary speeds the voltage generated is too low 



