Current transformer and transformer similar is also based on the principle of electromagnetic induction, transformer is to change the voltage and current transformer is to change the current. 

Current transformer connected to the current measured winding (turns number for N1) , known as the primary winding (or primary winding, Primary Winding) ; measuring instrument connected to the winding (turns number for N2) known as the secondary winding (or secondary side winding, secondary winding) . The current ratio between the primary winding current I 1 and the secondary winding current I 2 of a current transformer is called the actual current ratio k. The current ratio of a current transformer operating at rated current is called the rated current ratio of a current transformer and is expressed by Kn.