196 APPLIED SCIENCE 



former the two ends of the various armature coils are con- 

 nected in a ring. As the armature travels past the poles 

 of the field magnet, the armature coils cut through the mag- 

 netic field in opposite directions. This produces a flow of 

 current in the coils which reverses as the particular wire 

 passes each pole. The current is collected by means of the 

 rings, and is transmitted through the circuit as a series of 

 rapidly oscillating pulsations. It is necessary to have or 

 maintain the magnetic field of an alternating current dynamo 

 in a constant condition; that is, the lines of force must 

 always travel between the poles in a constant direction. 

 To attain this result, the field must be excited (receive its 

 power) from a dynamo generating a continuous current. 



239. Care of Dynamo. A dynamo to run properly must 

 be kept clean and dry. The parts that require the greatest 

 care are the commutators and brushes. The commutator 

 should be kept clean by wiping it with a, hard cotton cloth. 

 The occasional application of a little vaseline tends to di- 

 minish friction between the commutator and brushes. Oil 

 should never be used for this purpose. As the commutator 

 becomes roughened with age, it should be smoothed by hold- 

 ing fine sandpaper against it while the machine is revolving. 

 If the commutator gets out of true (out of adjustment), it 

 must be turned down in a lathe. If it becomes wet, the 

 insulation of the armature and field coils will be injured or 

 destroyed, because in such a case resistance between the 

 frame and the electrical part becomes low. A commutator 

 in which this trouble occurs is said to be " badly grounded." 

 There should be an insulation resistance of 10,000,000 ohms. 

 The bearings of a dynamo require no more attention than the 

 bearings of any other machinery. 



