CHAPTER V. 

 TRANSFORMERS. 



EXPERIMENT 5-A. Preliminary Study and Operation of a 

 Transformer. 



PART I. INTRODUCTORY. 



I. A transformer consists of three elements : a core of lami- 

 nated iron; and a primary and a secondary winding upon this 

 core. The two windings are insulated from each other and 

 usually from the core ; they are in close proximity to each other 

 or are so inter-spaced that practically all the flux which passes 

 through one must pass through the other i. e., there is the least 

 possible magnetic leakage. 



The transformer is used on alternating current circuits to 

 increase or step-up the voltage, or to decrease or step-doivn the 

 voltage, in the ratio of the number of primary to secondary turns 

 (Si'.Sz); there is a corresponding opposite change in the cur- 

 rent in the ratio S 2 : S lt an increase in voltage being accompanied 

 by a decrease in current, and vice versa. It is chiefly the trans- 

 former which makes alternating current superior to direct cur- 

 rent for power transmisssion, for it makes possible a high poten- 

 tial on the transmission line, with consequent copper economy 

 (50, Exp. 6-A), and any desired lower potential at the gen- 

 erating and at the receiving apparatus. 



2. In operation, the primary winding is connected to an alter- 

 nating current supply (see Fig. i). A current flows in the 

 primary which magnetizes the core, '. e., it sets up an alternating 

 magnetic flux which induces an electromotive force in the second- 

 ary winding and, when the secondary is closed through a resist- 



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