Ref. the yard hooter. It also had other uses than a time signal, we of the health physics department understood that a code existed whereby certain states of alert could be notified during a nuclear alert, such as a reactor core meltdown or release of fissile airborne contamination. As a lowly health physics monitor I was not privy to the code but local police and fire brigades had copies so, no matter where they had been deployed to, in or around the yard, the state of play ref. the incident could be broadcast to those who held a copy of the code.