 pressure from the voice vibrating against the diaphragm change the com- pression of the carbon granules. This varies the resistance of the transmitter. The transmitter has two contacts that are in- sulated from each other. Current can only flow through the carbon granules. As sound pressure from the voice presses against the diaphragm, the carbon is more closely compressed within the chamber. Compressing the carbon granules low- ers the resistance of the transmitter resulting in more current flow through the transmitter circuit. When the pressure on the diaphragm is released, it momentarily snaps out farther than its original position. The carbon is under less pressure than normal and the resistance of the transmitter is momen- tarily greater. The current flow decreases. The Telecom Handbook 35 15 THE TELEPHONE The diaphragm of a transmitter is made of lightweight phosphor bronze, duraluminum or a similar material. Either an extra inner cone of the same material strengthens the cen- ter or it is corrugated to act as a stabilizer. The flexible outer edge is securely clamped in the transmitter housing. This de- sign enables the diaphragm to move in and out at the center like a piston. Since the diaphragm is sensitive to sound waves, the carbon granules are compressed and released as the cor- responding pressure from the sound wave's changes. The telephone transmitters in use today are, in principle, like the ones invented more than 100 years ago by Thomas Edison. Many modern electronic telephones use real micro- phones connected to related speech processing equipment to vary the line current. Small microchips allow economy and space saving, enabling inexpensive, high quality "throwaway" telephones. The output now generated by microchip-based telephones must emulate the same variations created by the carbon granule type of transmitter. What is known as the basic 500 set, a single-line tele- phone like the one that was in use in most homes, has dictated the industry's electrical standard for the telephone instrument