The Functioning of the Carbon Granules

The resistance decreases with an increase of the area of the contact. It is also thought that mechanical forces on the granules establish new contacts as well as change the areas of those already formed.

A special type of carbon is used for the granules, and these must be specially selected as to size and quality. They must be free from corrosion and of high resistance, and their contact resistance must be sensitive to pressure changes. If the granules are not of good quality or if they are not carefully selected, they readily become packed and the transmitter becomes insensitive. Packing may also be caused by the entrance of moisture, by overheating, or by other misuses. Slightly jarring the transmitter will usually remedy this condition.

Figure 1. Cross section of a Western Electric carbon-granule transmitter. Although this type will gradually be replaced by those of later design, many transmitters of this general type will remain in useful service for years.

If the transmitter of Fig. 1 is held at an angle, the granules tend to fall away from the electrodes, and this may completely open the circuit. It is important that the transmitter be held close to the speaker's lips when it is being used. One advantage of the modern handset telephone is that, if the receiver is in position for hearing, the transmitter is in the correct position for talking.

When the current through the carbon transmitter is increased beyond a certain value, the points where the currents enter and leave the granules will become hot and arcing will take place. A hissing or frying sound¹⁰ will be heard, and the so-called frying or burning point of the transmitter has then been reached. The normal current taken by telephone transmitters is about 0.1 ampere. In certain water-cooled carbon-granule transmitters of historical interest 15 amperes have been carried continuously.⁷