Powdered graphite microphone, 1878-1890

Microphone consisting of small tube filled with (apparently) powdered graphite with means for adjusting contact pressure of the particles, probably made by David Edward Hughes, England, 1878-1890.

The development of the microphone owes much to David Edward Hughes (1831-1901). Building on the work of Willoughby Smith and Sir William Thomson, Hughes attempted to detect changes in the resistance of wire with sound. He discovered that these changes occurred only when his stretched test wire broke and when he touched the ends together. He found that light but constant pressure was the only essential and in this experiment tried powdered carbon with means for adjusting contact pressure. Hughes did not patent his discoveries, meaning that subsequent inventors were able to make use of them without giving Hughes the credit.