Carbon microphones are only seen on extremely old electronics, such as ancient telephones. The sound quality is poor, especially compared with all the newer technology, but the carbon mike is very simple to construct.

The concept behind how a carbon mike works is rather straightforward:

                       Tube   _________________________
                               ___________ |X|.:.:.:|X||
                              |           ||X|:.:.:.|X||
                              | PLUNGER   ||X|.:.:.:|X|| Back wall of tube
                                            |        |
                                            |        |
                                            |+      -|

The microphone consists of a hollow tube. A plunger is inserted into the tube, and is butted against a movable diaphragm with a coil, represented by the |X| in the diagram. The dots between the diaphragms represent carbon. Carbon allows an electrical current to pass, albeit with some resistance. Carbon also has the capability of changing the amount of current depending on how compacted it is. As sound waved forced the plunger into the tube, it would push the first diaphragm into the carbon, which would change the overall resistance of the circuit between the two diaphragms. As the soundwave allowed the plunger to return to its static position, the carbon would be less compressed, which would again change the resistance of the carbon and allow a smaller amount of current to flow. The sound wave would be, in effect, duplicated as a changing current flow between the diaphragms.

Carbon mikes can be made relatively easily, and make excellent science fair projects.