Carbon microphone facts for kids
The carbon microphone is a special type of microphone. It changes sound into an electrical audio signal. Think of it as a translator for sound! It was also known as a carbon button microphone or carbon transmitter.
This microphone has two metal plates with tiny pieces of carbon in between them. One plate is very thin. It faces the person speaking and acts like a diaphragm. When sound waves hit this thin plate, it vibrates. This vibration pushes on the carbon pieces.
When the carbon pieces are pushed closer, their electrical resistance changes. It becomes lower. A steady direct current flows through these carbon pieces. As the resistance changes, the electric current also changes. This creates an electrical signal that matches the sound waves. This signal can then travel through wires.
For many years, carbon microphones were very important. They were used in telephones until the 1980s. They helped send voices over long distances. They were also used in public address systems to make voices louder. However, these microphones had some limits. They could only pick up a narrow range of sounds and sometimes made a lot of electrical noise.
Before powerful vacuum tube amplifiers became common in the 1920s, carbon microphones were the best way to get strong audio signals. They were cheap and produced a good signal for telephones. Even today, old telephones with carbon microphones can still work without changes. Early AM radio also used them. But their limited sound quality meant other microphones soon replaced them for radio. Still, they were used for many years in simpler sound systems, military gear, and amateur radio.
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The Story of the Carbon Microphone
The first microphone that could properly send voices over a telephone was the carbon microphone. Several inventors worked on it around 1878. These included David Edward Hughes in England and Emile Berliner and Thomas Edison in the United States.
Even though Edison received the first patent in 1877, Hughes had shown his working device to many people earlier. Most historians believe Hughes was the true inventor. Hughes' device used loose carbon granules. When sound waves vibrated a plate, it pressed on these granules. This changed their electrical resistance, creating an accurate electrical copy of the sound. Hughes also created the word "microphone." He showed his invention by making the sound of insects scratching much louder. Unlike others, Hughes chose not to patent his invention. He wanted it to be a gift for everyone to use.
In America, Edison and Berliner had a long legal fight over who owned the patent rights. Eventually, a court decided that Edison had invented the technology first. The court stated that Edison was ahead of Berliner in sending speech. It also said that using carbon in a transmitter was Edison's invention.
The carbon microphone was a very important step for modern microphones. It was key to the growth of telephones, radio broadcasting, and the music recording industry. Carbon microphones were widely used in telephones from the 1890s until the 1980s.
Carbon Microphones as Amplifiers
Carbon microphones could also make sounds louder, acting like amplifiers. This was very useful in early telephone repeaters. These devices helped make long-distance phone calls possible before vacuum tube amplifiers existed.
In these repeaters, a magnetic telephone receiver (which turns electrical signals into mechanical movement) was connected to a carbon microphone. Because a carbon microphone changes a current flowing through it, it could boost weak signals. This allowed the signals to travel further down the telephone line.
These carbon amplifiers were mostly replaced once vacuum tubes were developed. Vacuum tubes offered even more amplification and better sound quality. However, carbon amplifiers were still used in the 1930s for portable devices like hearing aids. For example, the Western Electric 65A carbon amplifier was very small and light. These carbon amplifiers did not need the heavy batteries that vacuum tube amplifiers required. By the 1950s, tiny vacuum tubes, and then transistors, replaced carbon amplifiers in hearing aids.
You could even hear the amplification from carbon microphones through feedback. If you put the earpiece of an old "candlestick telephone" too close to its carbon microphone, it would make a loud squealing sound. This showed how much the microphone could boost the signal.
First Uses in Radio Broadcasting
Early AM radio transmitters used carbon microphones to add voice to the radio signal. In 1906, Reginald Fessenden made the first long-distance voice transmissions. He used a water-cooled carbon microphone to send a continuous radio wave from an Alexanderson alternator to the antenna. Later radio systems used carbon microphones to control the signal in vacuum tube oscillators. This helped create the modulated radio waves that carried sound.
Where Are Carbon Microphones Used Today?
Today, you might still find carbon microphones in older telephone systems. Their use depends on the area and country. While many manufacturers have stopped making them, some are still in demand for specific uses. For example, the Shure 104c microphone was still popular in the late 2010s. This was because it worked well with existing older equipment.
Why Carbon Microphones Are Still Useful
The main benefit of carbon microphones is their ability to create strong audio signals from very low electrical power. They don't need extra amplifiers or batteries to work.
A carbon microphone can actually increase the power of a signal. You can see this by connecting a battery, a microphone, and an earphone in a series. If you touch the microphone and earphone together, the system will start to make a sound, or oscillate. This happens because the power gain around the loop is greater than one.
These microphones are especially useful in remote places with very long telephone lines. The long wires can cause the electrical power to drop significantly. Most modern electronic telephones need at least three volts of direct current to work. They would stop working in such conditions. But carbon microphone telephones can still work with just a tiny fraction of a volt.
Carbon-based telephone systems are also very tough. They can resist damage from strong electrical surges, like those caused by lightning strikes. They are also resistant to powerful electromagnetic pulses. Because of this, they are still kept as backup communication systems in important military locations. They provide a reliable way to communicate even in extreme conditions.
