The weight of the upper plate affords an initial pressure which is varied by speaking into the mouth of the vessel.
The carbon-block may be replaced by a disk of cloth, the pores of which have been filled with pulverized black-lead. By this treatment the cloth becomes slightly conductive. The instrument thus modified is shown in Fig. 3.
In Fig. 4 the pulverized plumbago, P, is floated upon mercury, M, and is compressed between the surface of the mercury and a metallic block fastened to the centre of the diaphragm.
Still another form of the Edison transmitter is shown in Fig. 5. The carbon, C, rests upon the diaphragm, which in this instrument is an horizontal plate forming the top of a vocalizing chamber, the mouth-piece being at the side.
Three fine cords attach the carbon to the framework of the diaphragm, and prevent it from being displaced when the diaphragm is vibrating. In appearance this instrument resembles the Reiss telephone, and in principle it would be much the same were it not that, in vibrating, the carbon never actually leaves the plate upon which it rests, but simply for an instant releases its pressure. It is evident that the resistance of the circuit depends upon the electric connection between the carbon and the diaphragm, and that this connection depends