Pack of 8 CT (Current Transformer) coils for RR-CirKits Block Occupancy Detector (BOD-4/BOD-8).

These 100:1 toroid coils sense the track current on leads passing through the hole in the middle of the coil. Each coil must be connected to the occupancy detector using "twisted pair" wiring, such as is found in CAT-5 network cable.