A current transformer senses the fluctuations in the magnetic field and converts the value to an AC reading, which shows up on your display. Clamp meters are great for high AC currents, which make them an ideal tool to have around. If you're familiar with how direct currents work, you might wonder how a clamp meter would help you in a situation when you need to measure DC currents instead of AC. Clamp meters that can measure DC current use the Hall Effect to give you a correct measurement. http://www.gvda-instrument.com/electronic-instruments/digital-clamp-meter/