The Fissure Detector depends for its operation upon the fact that electric current flowing through a rail is compelled to pass around any fracture, inclusion, or separation in the metal. In the detector car, the power plant for the detector equipment consists of a specially designed double commutator 6,000-ampere, 2-volt DC generator directly coupled to a 50 hp engine. Current from the generator is carried by heavy busses to brush units located on either side of the car. In operation the car passes along the track at about 6 mph, with 2,000 to 3,000 amperes flowing through each rail. The current is sensed by the pickup unit, amplified and sent to a recording table, which consists of a moving roll of paper and nine pens (like a polygraph or lie detector). The top pen (landmark pen) is marked by the car's operator to indicate mileposts and other landmarks, the middle two pens record angle bars (rail joints). When the car passes over a fissure, and indication appears on the record. Three recorder pen relays are connected to the output of the amplifier and adjusted to different values of plate current, thus giving an indication of the size of the defect. Other relays are provided to operate paint guns and the defect area is automatically marked with a spot painted on the rail. The operator notes the record, sees the spot painted on the rail, and stopping the car, backs up for a hand test, applying 1,500 amperes to the suspected spot. Examination of the spot with a galvanometer can determine the size of the fissure within a few percent. The car can also find split heads, horizontal fissures, compound fissures, pipes, cracked webs, broken bases and other defects.