Not all inventions — scientific or otherwise — go according to plan. Some are monumental failures, while others are accidental successes. One such success came about in the 1930s when Swiss physicist Walter Jaeger lost a close childhood friend to toxic gas and was determined to invent some type of device that would prevent future accidents. He created a prototype and expected the gas entering the sensor to bind to ionized air molecules and thereby alter an electric current in a circuit of the instrument. However, small concentrations of gas didn't affect the sensor's conductivity. Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current. Unlike poisonous gas, the smoke particles from his cigarette were able to alter the circuit's current. Jaeger's experiment was one of the developments that paved the way for the modern smoke detector, and in 1951 the first ionized smoke detectors were sold in the U.S. to commercial and industrial facilities. It wasn’t until 1965 that a residential, battery-powered smoke detector was designed that could be easily installed. It cost $125 ($984 today). In 1995, the 10-year lithium-battery-powered smoke alarm was introduced for $60. Today, they sell for $19.95.
