Final answer:
A smoke alarm is a stand alone device that includes an electric circuit with metal plates and a radioactive element. When smoke particles enter the detector and interrupt the flow of current, an alarm is triggered.
Step-by-step explanation:
A smoke alarm is a stand alone device comprised of an electric circuit that includes two metal plates about 1 cm apart. The circuit is powered by a battery that creates a voltage between the plates. The smoke alarm also contains a small disk with a tiny amount of the radioactive element americium. The radioactivity of americium ionizes the air between the plates, causing a small current to flow. When smoke particles enter the smoke detector, they interrupt the flow of current, triggering a loud alarm.