The advance that was brought about by Edison's carbon transmitter will be more apparent if we glance first at the state of the art of telephony prior to his invention.

Bell was undoubtedly the first inventor of the art of transmitting speech over an electric circuit, but, with his particular form of telephone, the field was circumscribed. Bell's telephone is shown in the diagrammatic sectional sketch (Fig. 1).

In the drawing M is a bar magnet contained in the rubber case, L. A bobbin, or coil of wire, B, surrounds one end of the magnet. A diaphragm of soft iron is shown at D, and E is the mouthpiece. The wire terminals of the coil, B, connect with the binding screws, C C.

The next illustration shows a pair of such telephones connected for use, the working parts only being designated by the above reference letters.

It will be noted that the wire terminals are here put to their proper uses, two being joined together to form a line of communication, and the other two being respectively connected to "ground."