THE TRANSFORMER 



301 



Because of this neutralization of currents, the auto- 

 transformer is very economical in the use of copper, especially 

 when the secondary and primary voltages are nearly the same. 



76. The Constant-current Transformer. For arc light- 

 ing, the high voltage series system has proved to be 

 commercially successful and most arc light systems are of 

 this type. The arc lamps used in such work are of the 

 constant current type and requires a transformer which sup- 

 plies a constant secondary current. 



Constant Current System Requires Variable Voltage. 

 In the operation of a series arc system it often happens 

 that some lamps go out, for some reason or other, and of 

 course this changes the resistance of the load circuit. If 

 the voltage impressed on the 

 circuit were constant, the 

 current would rise and fall as 

 the number of lamps burn- 

 ing is decreased or increased 

 and the lamps would operate 

 poorly. Hence there is re- 

 quired in the station a trans- 

 former the secondary voltage of 

 which varies with the number 

 of lamps in circuit. 



Operation of the Constant- 

 current Transformer. The 

 constant-current transformer 

 accomplishes this variation 



by a variation of the leakage flux between the primary and 

 secondary. The secondary coil is made movable and the 

 primary fixed and the leakage flux will evidently depend 

 upon the separation of the two. A view of this type of trans- 

 former is given in Fig. 193. (This type is sometimes called 

 the " tub " transformer.) The secondary coil is suspended 

 and nearly counterbalanced so that but little force is re- 

 quired to move it up, or to separate it from the primary. 



FIG. 193. View of a Constant- 

 current Transformer. General 

 Electric Co. 



