HUNNINGSCONE

The early telephones used the same electromagnetic element for transmission and reception. A more sensitive transmitter was invented by the Reverend Henry Hunnings in 1878. The Hunnings microphone used loosely packed carbon granules between a thin conducting diaphragm and a metal housing. The electrical resistance between the diaphragm and the housing varies as the carbon granules move when the diaphragm vibrates. 

Wilhelm Deckert of Austria devised a modification, known as the Hunningscone transmitter. Both electrodes were made of carbon. The back electrode was scored to produce a series of pyramids or cones. The cones limit the carbon granules from running downhill under gravity. Still, the transmitters can be rotated on their mounting to counteract packing. 

I have an early telephone, equipped with a Hunningscone transmitter, probably made by Peel-Conner around 1900.