When he recorded an experiment that brought to light properties of nature which were previously unknown he recorded them as "phenomenon."
One such phenomenon was the behavior of carbon in the form of lamp black or powdered graphite. Edison's famous "carbon button" was made from lamp black which he carefully gathered from the chimneys of a large number of smoking kerosene lamps. Earlier, Edison had found that such finely divided carbon could be used to vary the strength of current flowing through a wire. He learned this in the year 1873, when he was trying to find a way to speed up the transmission of telegraph messages over a long under-ocean cable.
Anything in the wire that slows the movement of the electrons is called resistance. The resistance of a thin wire is greater than that of a thick wire. The resistance of a long wire is greater than that of a short wire. The resistance of a transoceanic cable was tremendous.
To conduct his cable experiments, Edison tried to simulate in his laboratory the conditions of a 3,000-mile-long cable. To produce the resistance that could compare, he pressed finely ground graphite into glass tubes and inserted wires in the ends of the tubes. By putting many of the tubes end to end, he was able to approximate the resistance of the cable. Edison tried, to no avail, to use the graphite-filled tubes in his experiments to test what would happen in the cable under various conditions. It did not work.
The tubes did give the resistance he needed. But he found that he could not keep the resistance constant. The slightest pressure on the end of the tube, even a vibration in the wires, varied the resistance.When Edison saw that the arrangement would not do, he laid it aside. 4 years later he remembered the "phenomenon" of the behavior of carbon and put it to another kind of use.
He was then at work trying to find some way to transmit the vibrations caused in a telephone diaphragm by the human voice. He thought of the curious behavior of the graphite in the glass tubes. He recalled that vibrations of the wire had altered the resistance of the graphite to electric currents.
Edison now experimented with graphite in various forms. He used a stick of graphite adjusted so that it would touch lightly on a spring attached to a telephone diaphragm. When a person spoke into the telephone, the diaphragm would vibrate, causing a change in pressure on the spring and so on the graphite. This caused a variation in the current passing through the graphite.
Edison found that he could get a tremendous sensitivity using the different forms of graphite with the telephone. The instrument would pick up the faintest sounds--a whisper, the touch of a finger, the foot-step of a fly, even a softly exhaled breath. This was the basis for the invention of the microphone. Reproduction of speech was not distinct. The instrument would blare but it would not speak clearly.
One day Edison happened by be distracted from his work by the annoying smoking of his kerosene lamp. Glancing at the lamp, he noticed the intense black of the smoked-up chimney. Edison was curious about this deposit and as soon as the lamp had cooled a little he wiped off some of the black and examined it.
It was not long afterward that Edison gathered the black from many chimneys and pressed it into a mold to make the first carbon button. Soon he set up banks of lamps and put them all to smoking in what he called his carbon factory. He kept assistants busy gathering the lamp black and pressing it into buttons.
The carbon button played an important part of the history of radio. It started in a chimney in Tom Edison's lab.