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A SUPERSENSITIVE
CIGAR-BOX MICROPHONE

     Although Alexander Graham Bell is credited with the invention of the telephone, it was Thomas Edison who devised the first telephone transmitter that could be used over long distances. Unlike Bell’s limited-range instrument, Edison’s transmitter took advantage of a wonderful property of carbon: If a loose pack of carbon particles is squeezed, the electrical resistance of the pack decreases. In other words when current is passing through the pack, more current will flow when pressure is applied.

     Edison had the idea that voice waves could apply that pressure. And he was right. In the carbon transmitter he perfected, loud voice sounds (upon striking the carbon particles and compressing them) caused larger currents than did quieter sounds. These variations in current traveling down a transmission line regulated a receiver at the other end of the line, and that receiver reproduced the sounds of the speaker’s voice. This use of carbon in a telephone is still practiced today. Edison’s carbon-particle device, then, was the forerunner of the modern telephone transmitter and the microphone used in radio broadcasting.

     The cigar-box microphone, shown here, is similar to Edison’s in at least one respect: It is a closed-circuit system, which means that current is constantly flowing. Edison’s first “speaking telegraph transmitter” (patent no. 474,320) included this important concept. Bell’s instrument did not, which is one of the reasons its range was limited to only a few miles. However, the cigar-box “mike” is not a carbon-particle transmitter, even though it uses carbon. It is a loose-contact mike. It won’t give anywhere near the sound quality that Edison’s did. Nevertheless, it is an extremely sensitive detector of sound and one that can be fun to make and experiment with.

How Does Our Mike Work?

     Being a loose-contact detector, the cigar-box microphone has the same high sensitivity to vibrations as insecure electrical connections. You’ve no doubt noticed how easily a loose light bulb flickers when someone passes by. So it is with our mike (see circuit diagram). The carbon electrodes loosely support the pencil-lead rod. The slightest vibration, like from a sound, will disturb the rod. When the circuit is closed and current is flowing through the headphones, this disturbance changes the current flow. The headphones respond to these changes and, hence, tend to imitate the sound.