fire alarm

a signal that warns that a fire has started.

a bell, buzzer, siren, horn, etc., that provides such a signal.

Origin of fire alarm

An Americanism dating back to 1840–50

A crowd had congregated around the fire-alarm box, and would not let the men pull it.

Report of the Committee Appointed to Investigate the Railroad Riots in July, 1877 |Various
It was just a little "fire-alarm drill" to keep the reserve troops up to the high-water mark of efficiency.

The New York Times Current History: the European War, February, 1915 |Various
"Fourth Ward, Bottom Level," a voice from the fire-alarm speaker said.

Four-Day Planet |Henry Beam Piper
That fire-alarm thing scared him crazy—he's usually so good!

Queed |Henry Sydnor Harrison

Hearing the fire-alarm and looking out, he was astounded to see that it was the freight-house burning.

Dynamite Stories |Hudson Maxim

fire alarm

a device to give warning of fire, esp a bell, siren, or hooter

a shout to warn that a fire has broken out