Miniature current transformers are also called "instrument current transformers". (The "instrument current transformer" has a meaning that the multiple current ratio precision current transformer used in the laboratory is generally used to expand the range of the instrument.) Current transformers are similar to transformers and work according to the principle of electromagnetic induction. The transformer transforms Voltage and current transformers transform currents. The winding of the current transformer connected to the measured current (the number of turns is N1) is called the primary winding (or primary winding, primary winding); the winding (the number of turns is N2) connected to the measuring instrument is called the secondary winding (or the secondary winding) Winding, secondary winding).