Current probe are toroidal transformers designed to measure RF currents on active power lines or other conductors. They are used for RF conducted immunity testing.

A current probe is used as a “pick-up” device for measuring RF current in single conductors or cable bundles.

Direct connection to the conductor carrying EMI current is not necessary, since the probe may be opened for insertion of the conductor into the window of the toroid and then closed again to form a toroidal transformer with the conductor acting as a one-turn primary.