Abstract: 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 comprising 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 said first contact for maintaining electrical contact with the pipe as the vehicle moves through the pipe, and a voltage reading device connected to said first contact and said second contact for measuring the voltage between said first contact and said second contact as the vehicle moves through the pipe.