Despite major bombings that have rattled the nation, and fears of rising violence as American troops withdraw, Iraq’s security forces have been relying on a device to detect bombs and weapons that the United States military and technical experts say is useless. The small hand-held wand, with a telescopic antenna on a swivel, is being used at hundreds of checkpoints in Iraq. But the device works “on the same principle as a Ouija board” — the power of suggestion — said a retired United States Air Force officer, Lt. Col. Hal Bidlack, who described the wand as nothing more than...