Only two wires are required to send the signal and also supply power to the sensor • Signal may be analog or digital • The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop • Also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). • Such instruments are used to measure pressure, temperature, flow, pH … • A current loop can also be used to control a valve positioner or other output actuator. • An analog current loop can be converted to a voltage input with a precision resistor.