A thin diaphragm of magnetic material is suspended from the inside edges of the magnetic ring so it is positioned between the magnet and the coil. The magnetic field created by the magnet changes with the current in the coil and makes the diaphragm vibrate. The vibrating diaphragm creates sound waves that replicate the sounds that were transformed into electricity by the other person's transmitter.


C. Alerter

The alerter in a telephone is usually called the ringer, because for most of the telephone's history, a bell was used to indicate a call. The alerter responds only to a special frequency of electricity that is sent by the exchange in response to the request for that telephone number. Creating an electronic replacement for the bell that can provide a pleasing yet attention-getting sound at a reasonable cost was a surprisingly difficult task. For many people, the sound of a bell is still preferable to the sound of an electronic alerter. However, since a mechanical bell requires a certain amount of space in the telephone to be effective, smaller telephones mandate the use of electronic alerters.


D. Dial

The telephone dial has undergone major changes in its history. Two forms of dialing still exist within the telephone system: dial pulse from a rotary dial, and multifrequency tone, which is commonly called by its original trade name of Touch-Tone, from a push-button dial.

In a rotary dial, the numerals one to nine, followed by zero, are placed in a circle behind round holes in a movable plate. The user places a finger in the hole corresponding to the desired digit and rotates the movable plate clockwise until the user's finger hits the finger stop; then the user removes the finger. A spring mechanism causes the plate to return to its starting position, and, while the plate is turning, the mechanism opens an electrical switch the number of times equal to the dial digit. Zero receives ten switch openings since it is the last digit on the dial. The result is a number of "dial pulses" in the electrical current flowing between the telephone set and the exchange. Equipment at the exchange counts these pulses to determine the number being called.

The rotary dial has been used since the 1920s. But mechanical dials are expensive to repair and the rotary-dialing process itself is slow, especially if a long string of digits is dialed.