22 ELECTRICAL MACHINERY 



In the armature winding of a dynamo-electric machine 

 some of the conductors cut the magnetic flux and others 

 (as for example, the end connections) do not cut any flux 

 and so generate no e.m.f. when the armature is rotated. 

 A distinction is sometimes made by calling those con- 

 ductors of the armature in which an e.m.f. is generated 

 inductors; the rest of the wires making up the winding 

 are called conductors. 



Direction of the Induced E.M.F. As has been said, an 

 induced e.m.f. is always produced in a conductor where 

 the conductor is so moved that it cuts magnetic flux. The 

 direction of this induced e.m.f. is dependent upon the 

 direction of the magnetic flux and the direction of the 

 motion. A convenient rule which gives the direction of 

 the induced e.m.f. is as follows: Place the index finger of 

 the right hand in the direction of the magnetic lines of force 

 and the outstretched thumb in the direction of the motion; 

 the middle finger, held at right angles to the index finger , 

 mil then indicate the direction of the induced e.m.f. This, 

 of course, will also be the direction of the current set up 

 in the conductor on closed circuit. 



11. Force Exerted between a Magnetic Field and a 

 Conductor Carrying Current. If a conductor is placed in 

 a magnetic field (not parallel to the direction of the field) 

 and current is passed through the conductor, it will be 

 found that the conductor and magnetic field so act on one 

 another that a force is developed which tends to cause the 

 conductor to move in a direction perpendicular to the 

 direction of the magnetic field and to its own length. 



Direction of Force. If the direction of the field is hori- 

 zontal, then the conductor will be urged in a vertical direction 

 when current is passed through it. Whether this force 

 will be up or down will depend upon the direction of the 

 current and the direction of the magnetic field. A rule 

 which gives the direction of the force is as follows: If the 

 index finger of the right hand is held pointing in the direction 



