The diaphragm is a piece of thin sheet-iron cut in the form of a circle two and one-half inches in diameter. A small hole is bored through the center of this. One of the carbon buttons is fastened to the center of the diaphragm with a small screw and a nut. Cut out a strip of flannel or thin felt, nine-sixteenths of an inch wide and three and one-half inches long. Around the edge of the carbon button mounted on the diaphragm, bind this strip with silk thread in such a manner that the strip forms a cylinder closed at one end with the button. Fill the cylinder with polished carbon telephone transmitter granules to a depth of about one-eighth of an inch. These granules will have to be purchased from an electrical supply house. They are finely polished small carbon balls, much like birdshot in appearance. Slip a long machine-screw through the hole in the second carbon button and clamp it in place with a nut. Then place the carbon button in the cylinder so that it closes up the end. The space between the two buttons should be about three-sixteenths of an inch. Bind the flannel or felt around the button with a piece of silk thread so that it cannot slip out of place. The arrangement of the parts should now be the same as that shown by the cross-sectional drawing in the upper right-hand corner of Figure 148. The complete transmitter is assembled as shown in the lower part of Figure 148. A small tin funnel is fitted into the hole in the face-plate, _A_, to act as a mouthpiece. A screw passes through the back, _B_, and connects to the diaphragm. The screw is marked "_E_" in the illustration. A binding-post is threaded on the screw so that a wire may be easily connected. The screw passing through the back carbon button also passes through a hole in the wooden back, and is clamped firmly in position with a brass nut so that the button is held very rigidly and cannot move. The front button, being attached to the diaphragm, is free to move back and forth with each vibration of the latter. [Illustration: Fig. 148.—A Home-made Telephone Transmitter.] 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.