Description Wheatstone Bridge. ZM-4B/U Resistance Bridge in gray metal case. Supply: Internal Battery, Serial: 219, For General Use. Manufactured by Shallcross Manufacturing Co. in Collingdale, Pennsylvania, for Navy Department-Bureau of Ships. Contract: NODS(?)-52316 4 feet on bottom and 4 feet on back, and all are intact and fully functional. Operating instructions inside of case are legible, but very stained, along with smudges. This type of instrument is used to measure the unknown resistance of an electrical component. It includes 5 decade resistance selector switches, a galvanometer with zeroing knob, binding posts and selector switches for mode and power supply. Highly precise, this bridge can be used to identify and locate grounds and shorts, to measure loop resistance or resistance of unknown components. In conjunction with other test equipment, it can aid in accurately locating faults in very long cables.