The carbon granules should fill the space between the buttons three-quarters full. They should lie loosely together, and not be packed in.
When connected to a battery and a telephone receiver the current passes from the post, D, to the back button, through the mass of carbon granules into the front button and out at the post, E. When the voice is directed into the mouthpiece, the sound waves strike the diaphragm and cause it to vibrate. The front button attached to it then also vibrates and constantly changes the pressure on the carbon granules. Each change in pressure is accompanied by an immediate change in resistance and consequently the amount of current flowing.
Figure 149 shows a complete telephone ready for mounting on the wall. It consists of a receiver, telephone transmitter, bell, hook, and push-button. The bell is mounted on a flat base-board. The transmitter is similar to that just described, but is built into the front of a box-like cabinet. The box is fitted with a push-button at the lower right-hand corner. A simple method of making a suitable push-button is shown in the upper left-hand part of the illustration. It consists of two small brass strips arranged so that pushing a small wooden plug projecting through the side of the cabinet will bring the two strips together and make an electrical connection.
The "hook" consists of a strip of brass, pivoted at one end with a round-headed brass wood screw and provided with a small spring, so that when the receiver is taken off of the hook it will fly up and make contact with a screw, marked C in the illustration. When the receiver is on the hook, its weight will draw the latter down against the screw, D. The hook is mounted on the base-board of the telephone, and projects through a slot cut in the side of the cabinet.
Four binding-posts are mounted on the lower part of the base-board. The two marked B and B are for the battery.
Fig. 149.—A Complete Telephone Instrument. Two Instruments such as this are necessary to form a simple Telephone System.
That marked L is for the "line," and G is for the ground connection or the return wire.
Fig. 150.—Diagram of Connection for the Telephone Instrument shown in Fig. 149.