A Current Transformer is a form of sensor used to measure current through a wire or cable. Typically they will be used to sample a portion of current through a conductor and provide current feedback without connecting directly to the circuit, so they provide isolation too. But they are made of wire that will have reactance. Other current sensors like the Hall Effect do not have reactance and are much faster in response to actual changes in the conductor being measured, but are also much more expensive to use. This help?