On circuits newly bonded with two 46-in. galvanized iron wires a joint, the combined rail and bond wire resistance was found (on the L. & N.) to vary from .02 ohm per 1000 ft. of track on some circuits to .265 ohm on others, a difference of over 1300 per cent. This was rather puzzling. After a great many measurements had been made on different circuits it was found that no two measurements gave the same results, notwithstanding the fact that in many circuits the size of rail, length of bond wires, and age of bonding were exactly the same. On account of the bonding being new and the channel pins well driven, the contact between the bond wire and rail was above suspicion. The only other part of a track circuit that could possibly be the cause of this difference was in the contact between the angle bars and rails, and this later proved to be the case. Actual measurements made in the field proved that when the rail is new and the joint bolts tight, nearly all of the current flowing from rail to rail passes through the angle bars, whereas when the rails get old a coating of rust and dirt forms between the rail and angle bars, forcing practically all of the current through the bond wires. On most of the circuits measured on the L. & N. the combined rail and bond wire resistance was found to be less than .1 ohm per 1000 ft. of track, although many were found to be between .10 and .30 ohm per 1000 ft. It is interesting to note that two circuits were found bonded with two 52-in. iron wires, for which the combined rail and bond wire resistance measured .410 ohm, and that by adding two 40 per cent. copper clad bond wires to each joint the combined resistance was reduced to .144 ohm. The Growth of the Track Circuit Unfortunately there exists little or no data regarding the mileage of track circuits installed from the time the first installation was made by Dr. Robinson at Kinzua, Pa., and Irvineton up to about 1905. During the period between January 1, 1905, and September 30, 1906, the total automatic block signal mileage installed was 1,710.6, which brought the total up to 6,826.9 for the United States. Between September 30, 1906, and January 1, 1908, 3,976.1 miles of automatic signals were installed, which increased the above total to 10,803.0 miles.