Final answer:
A manual fire alarm is activated by using a fire alarm switch, which manually signals a fire emergency. Smoke detectors, using americium-241, automatically trigger an alarm when smoke disrupts the ion-based electric current between plates.
Step-by-step explanation:
A manual fire alarm can be activated by using a fire alarm switch. When you use a fire alarm switch, such as pulling a lever or pressing a button, it signals a potential fire emergency. This action is manual and does not depend on automatic detection like the presence of smoke or heat. Smoke detectors play a different role in fire detection, where they use a small amount of radioactive material, typically americium-241, to ionize air between two electrode plates. This ionization allows a small electric current to flow, which, in the presence of smoke, is disrupted, resulting in a current drop and triggering the alarm automatically. Therefore, while large amounts of smoke can activate an automatic smoke detector, it does not directly activate a manual fire alarm. Similarly, breaking the glass of a fire extinguisher casing or calling 911 will not activate a manual fire alarm system.