CHAPTER III. 

 THE OPERATION OF THE DYNAMO AS A GENERATOR. 



41. Constant voltage and constant current supply. When a 

 number of lamps, or motors, or both, are connected in parallel 

 with each other across the supply mains, it is necessary to main- 

 tain an unvarying electromotive force between the mains in order 

 that the various lamps and motors may operate independently of 

 each other. This method of supply is called constant voltage dis- 

 tribution. In this system a given lamp or motor is put out of 

 service by disconnecting it from the mains or by simply opening 

 its circuit. When a lamp or motor is put out of service in this 

 way, each remaining lamp or motor takes the same current as 

 before, and the generator delivers a total current which is less 

 than before by the amount of current which was taken by. the 

 disconnected lamp or motor. 



When a number of lamps connected in series are supplied with 

 current, it is necessary to maintain a constant current in this 

 circuit in order that the various lamps may operate independently 

 of each other. This method of supply is called constant ciirrcnt 

 distribution. In this system a given lamp is put out of service, 

 not by breaking its circuit, but by short-circuiting it, that is by 

 closing a switch which establishes a by -pass for the current. The 

 closing of this switch generally reduces the total resistance of the 

 circuit of lamps as a whole, and the maintenance of a constant 

 current in the circuit requires a decrease of the electromotive force 

 of the generator when lamps are put out of service, and an 

 increase of the electromotive force of the generator when addi- 

 tional lamps are put into service. 



The constant voltage system of supply is almost universally 

 employed nowadays, both for direct-current distribution and for 



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