Placard reads: "This cross arm once carried multiple pairs of wire from the control bunker at Trinity Site to ground zero. Located 30,000 feet (5.7 miles) south of ground zero, the control bunker was where the first atomic explosion was triggered. A signal was sent from the bunker over wire carried on these cross arms to the automatic sequencing device on top of the 100-foot tower where the bomb rested. Once the signal was sent, the first few seconds of the countdown proceeded automatically." Kudos to the White Sands Missile Range for their outstanding museum.