Current transformer is an instrument transformer used to measure the current flowing through the windings. Normally primary winding will be single wire and secondary will be many turns so the secondary measuring current will be less.
The current transformers can be used for metering purpose or for protection purpose. The output of the CT will be normally 5 or 10 amps. So according to the turn’s ratio will be arranged. Figure shows the arrangement of CT where the primary conductor carrying current induces the magnetic flux in secondary turns to produce the respective turns.