A system for measuring a voltage differential in a current-carrying pipe using a propulsion vehicle for conveying the system inside the pipe. The system includes a first contact for maintaining electrical contact with the pipe as the vehicle moves through the pipe, a second contact positioned in a spaced apart relationship from the first contact for maintaining electrical contact with the pipe as the vehicle moves through the pipe, and a voltage reading device connected to the first contact and the second contact for measuring the voltage between the first contact and the second contact as the vehicle moves through the pipe.