How do Smoke Alarms work?
Also known as ‘smoke detectors’, smoke alarms are devices which detect ‘smoke’ and then issue a warning. They have been around for quite sometime, but it was only recently, in the past 30 years, that they became commonplace. The first fire alarm was invented by Francis Robbins Upton, an associate of Thomas A. Edison, in 1890. The answer to the question, how does a fire alarm work, depends upon the kind of fire alarm at hand.
All fire detecting devices consist of the following essential parts: a sensor to detect smoke and a very loud electronic horn to warn the people. Based on the kind of detectors which are used, fire alarms can broadly be classified as:
Optical or photo-electric detectors:
The basic working scheme is like this: the alarm has a light source and a detector which are placed some distance from each other. A light ray continuously passes from the source to the detector through the intervening clean air. In case of a fire, smoke particles are produced and get mixed in the air; thus changing the intensity of the light ray which is passing through the air gap. The system picks up this change in light intensity and does what it has been programmed to do.
Ionization detectors:
These are a cheaper option than the optical fire alarms and much more sensitive. But it is this sensitivity which makes things complicated, as these fire alarms are prone to raising false alarms. This type of alarm has a chamber which contains a very small amount of a radioactive substance. The rays emitted from the radioactive substance contain alpha particles, which are in turn, conductors of electricity. When smoke enters the chamber where the alpha particles are, it reacts with them and changes the conductivity of the alpha particles. This raises an alarm.
