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    Home / Science lessons / How Light Bulbs & Telephones Work
    • How Light Bulbs & Telephones Work

      The Light Bulb & Thomas Alva Edison

      Perhaps one of the most important inventions of all time is the electric light bulb. We could get by with candles or lanterns in our homes, but imagine trying to shop at the mall, work in a large office complex, or travel at night by car or plane without electric lighting!

      Thomas Alva Edison, one of the developers of the modern light bulb, is also one of the most famous as well as prolific inventors in history. He patented over 1000 inventions in his lifetime. Edison was born in Ohio in 1847. As a child, he received less than a year of formal schooling, but was educated at home. His parents allowed him to set up a laboratory in their basement and his mother gave him books about chemistry and electronics. Edison credits his mother as being 'the making' of him.

      When he was 12, Edison got a job selling newspapers on a train that made day trips between his hometown of Port Huron, Michigan and Detroit. He took his laboratory along in the baggage car so that he would be able to experiment during layovers. This worked until one day some of his chemicals spilled and started a fire! Also while working on the train, Edison saved the child of one of the station masters, and as a reward was taught how to use a telegraph machine. He became a telegraph operator and began making improvements to the telegraph's functionality. He later invented a way for multiple telegraph messages to be transmitted simultaneously (rather than one at a time).

      As a young man, Edison moved to New York City and eventually established a laboratory. In 1877 he invented the phonograph, which used a record made of tinfoil to play back sound. In 1879 he created a successful incandescent light bulb. This was his hardest project - from 1877 to 1880, Edison and his assistants tried around 3000 experiments to perfect their light bulb design. By the end of 1880, Edison had produced a bulb that lasted 1500 hours. (This has a moral: don't worry if some of your own science experiments aren't a success the first time you try them!)

      Edison was not the first person to make a working light bulb: In the 1860s another English scientist, Sir Joseph Wilson Swan, began experimenting. He made a light bulb that used carbonized paper for a filament. But Swan lacked a strong enough vacuum inside the bulb; a design problem that he corrected at almost the same time Edison did. After a legal battle between the two inventors, Edison and Swan teamed up and created the company Ediswan to market their invention.

      How exactly do light bulbs work? For being such an important part of our lives, they have a fairly simple design. The base contains two metal contacts, which connect to an electrical circuit. These contacts are attached to two wires, which are in turn attached to the filament in the middle of the bulb (the filament is usually supported by a glass mount). A power supply sends an electric current from one contact to the other, traveling up through the wires and the filament. As the current passes through the filament, it 'excites'' the atoms that make up the filament material, causing them to give off energy in the form of heat and light. Edison tried thousands of different materials for his filaments, but most of them produced light for only a short time. He finally tried a carbonized cotton thread, which burned for many hours. Eventually, light bulbs were made using the metal tungsten for the filament. Tungsten works well because it has an unusually high melting point. This is important because metal must be heated to extreme temperatures to give off enough light.

      The danger of running electrical current through the filament is that the resulting high temperatures may cause the filament to combust, or catch on fire. This will only happen if oxygen is present, so Edison sucked all the air out of his glass bulbs to create a vacuum. This method worked fine to prevent combustion, but it allowed filament atoms to evaporate, shortening the life of the bulb. In modern bulbs, the glass is filled with an inert gas, such as argon. This prevents combustion, and also helps prevent tungsten atoms from evaporating.

      For over a century we have read and worked by the light of Edison's incandescent bulb. Other light sources, such as fluorescent bulbs and LED, have gained popularity in recent years because of their efficiency. The incandescent bulb releases most of its energy in the form of heat, not light, thus wasting electricity. Fluorescent and LED, however, are much cooler, and release most of their energy as visible light. These light sources have replaced the incandescent bulb for many functions.

      Although almost all of Edison's discoveries were technological rather than strictly scientific, his invention credits include an alkaline storage battery, microphone, mimeograph (a copy machine), and a kinetoscope for viewing moving pictures. He also created the first 'talkie,' a movie that had sound, using his kinetoscope and phonograph. He continued to improve not only the incandescent light bulb, but some of his other inventions like the phonograph as well.

      The Telephone & Alexander Graham Bell

      What if the only way you could communicate with someone living in another city was by mailing them a letter, one that might take weeks to get delivered? (Even with our modern technology, it still takes at least two weeks for mail to get to some countries.) The telephone is certainly one of the greatest inventions in the history of communication!

      The inventor of the telephone, Alexander Graham Bell, was born in Scotland in 1847, the same year as Thomas Edison. He went to university in Edinburgh and London, then immigrated to Canada in 1870 and to the U.S. a year later. There he used visible speech (a type of phonetic notation that shows the position of the throat, mouth, and tongue to make different sounds) to teach deaf-mute people how speak.

      When he was a young man, Bell became interested in transmitting speech, the way a telegraph transmitted messages. In 1874 he developed the idea for the telephone and successfully created and patented one two years later. (Just two hours after he applied for his patent, Elisha Gray filed his intention to apply for a patent for a very similar device.) His first transmitted sentence was to his assistant: 'Watson, come here; I want to see you." Bell demonstrated his telephone at the Philadelphia Centennial Expo and in 1877 organized the Bell Telephone Company.

      So how does this amazing invention work? How can you talk to someone miles away and hear them as though they were in the same room with you? It's actually a fairly simple process. At its most basic, a telephone consists of a switch, a receiver, and a microphone. The switch connects your phone to the telephone network; the receiver changes electrical signals into sound waves so you can hear the other person, and the microphone changes the sound waves you make into electrical impulses to send to the other end of the line.

      The microphone contains carbon granules and a thin metal diaphragm. When you speak, the sound waves (vibrations in the air) hit the diaphragm and cause it to move, compressing the carbon granules behind it. A louder sound will compress the granules more than a soft sound. When the carbon is compressed, an electrical current can flow through it more easily. A battery (either in your handset or at the telephone company) sends an electric current through your phone; when you speak, the compression of the carbon varies the intensity of the current.

      This process is reversed at the other end of the line by the receiver. The electrical current flows through an electromagnet in the receiver, producing a magnetic field. This magnetic field attracts the thin metal diaphragm in the receiver, causing it to move in and out. As it does so, it pushes and pulls the air, creating sound waves. These waves reach your ear with the same intensity as they were spoken, allowing you to hear what the other person said.

      The process hinges on the ability to have a variable electrical current. Unlike the telephone, a telegraph operates on a constant electric current - the messages are transmitted not by changing the current, but by repeatedly turning it on and off in a distinct pattern. Because the human voice doesn't start and stop sound waves in the same way as tapping Morse code does, the current must be able to adjust for variation in volume and frequency. The carbon microphone allows for this variation, giving us the ability to transmit speech electrically.

      Bell probably couldn't have imagined the kind of communication we have today. Not only do we have long-distance telephone networks all over the world, we also have cellular phones that use radio frequencies to allow us to talk wirelessly wherever we are. Communication was changed forever by the telephone and continues to develop rapidly today.

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