In a current transformer (CT), the primary current flowed through the primary circuit, typically a conductor or power line passed through the CT’s core. The primary current was the current that needed to be measured or monitored. CTs were designed to step down this primary current to a proportional, reduced secondary current. The secondary current was the output of the CT, and its magnitude was directly proportional to the primary current, as determined by the CT’s transformation ratio. This step-down process ensured that the secondary current was safe and manageable for measurement or use in various applications, such as metering, protective relaying, and control systems.