NO. 2 HISTORY OF ELECTRIC LIGHT SCHROEDER 69 



THE CONSTANT CURRENT TRANSFORMER FOR SERIES CIRCUITS 



About 1900 the constant current transformer was developed by 

 EHhu Thomson. This transforms current taken from a constant 

 potential alternating current circuit into a constant alternating current 

 for series circuits, whose voltage varies with the load on the circuit. 

 The transformer has two separate coils ; the primary being stationary 

 and connected to the constant potential circuit and the secondary 

 being movable and connected to the series circuit. The weight of the 

 secondary coil is slightly underbalanced by a counter weight. Current 

 flowing in the primary induces current in the secondary, the two 

 coils repelling each other. The strength of the repelling force depends 

 upon the amount of current flowing in the two coils. The core of the 

 transformer is so designed that the central part, which the two coils 

 s'lrround, is magnetically more powerful close to the prim.ary coil 

 than it is further away. 



When the two coils are close together a higher voltage is induced 

 in the secondary than if the later were further away from the primary 

 coil. In starting, the two coils are pulled apart by hand to prevent too 

 large a current flowing in the series circuit. The secondary coil is 

 allowed to gradually fall and will come to rest at a point where the 

 voltage induced in it produces the normal current in the series circuit, 

 the repelling force betwen the two coils holding the secondary at this 

 point. Should the load in the series circuit change for any reason, the 

 current in the series circuit would also change, thus changing the 

 force repelling the two coils. The secondary would therefore move 

 until the current in the series circuit again becomes normal. The 

 action is therefore automatic, and the actual current in the series 

 circuit can be adjusted within limits to the desired amount, by varying 

 the counterweight. A dash pot is used to prevent the secondary coil 

 from oscillating (pumping) too much. 



In the constant current transformer, the series circuit is insulated 

 from the constant potential circuit. This has many advantages. A 

 similar device, called an automatic regulating reactance was developed 

 which is slightly less expensive, but it does not have the advantage of 

 insulating the two circuits from each other. 



ENCLOSED SERIES ALTERNATING CURRENT ARC LAMPS 



The simplicity of the constant current transformer soon drove the 

 constant direct-current dynamo from the market. An enclosed arc 

 lamp was therefore developed for use on alternating constant current. 



