476 



FARM MOTORS 



The needle shows that a current has been induced which 

 has two directions of flow during each revolution of the 

 coil. This induced current is produced in exactly the 

 same manner in which currents are produced by 

 dynamos. 



636. Dynamos are machines for converting mechanical 



into electrical energy. They 

 cannot develop energy but 

 simply change the form 

 of the energy delivered to 

 them. Since they cannot 

 develop energy, the amount 

 of current delivered by 

 them is wholly dependent 

 upon the amount of me- 

 chanical energy supplied. 

 In principle the dynamo 



FIGi 35 6 consists of two parts : a 



magnetic field made up of 



electromagnets and a number of coils of wire wound upon 



an iron core forming an armature. 



637. Simple alternating-current dynamo. Consider the 

 single loop of wire ABCD (Fig. 356) as the armature 

 and the poles N and S as the magnets of a dynamo. With 

 the armature in the position it is shown there is no cur- 

 rent developed. The armature is for the instant moving 

 parallel to the magnetic lines of force and consequently 

 is cutting none of them. As the armature moves from a 

 position perpendicular to the lines of force to a position 

 parallel to them, the number of lines it cuts increases 

 until it reaches the perpendicular position, and from then 

 on until it has traversed 180 the number of lines cut de- 

 creases until none are cut. From this it is obvious that 

 with the armature in the first and last positions no cur- 



