One end of the bar is fitted with two thick fiber washers about seven-eighths of an inch in diameter and spaced one-quarter of an inch apart. The bobbin so formed is wound full of No. 36 B. & S. gauge single-silk-covered magnet wire. The ends of the wire are passed through two small holes in the fiber washers and then connected to a pair of heavier wires. The wires are run through two holes in the curtain-pole, passing lengthwise from end to end, parallel to the hole bored to receive the bar magnet. This bar magnet is then pushed through the hole until the end of the rod on which the spool is fixed is just below the level of the edges of the shell. The two wires are connected to binding-posts, _A_ and _B_, mounted on the end of the receiver. A hook is also provided so that the receiver may be hung up. The diaphragm is a circular piece of thin sheet-iron, two and one-half inches in diameter. It is placed over the shell, and the bar magnet adjusted until the end almost touches the diaphragm. The magnet should fit into the hole very tightly, so that it will have to be driven in order to be moved back and forth. The diaphragm is held in place by a hard-wood cap, two and three-quarter inches in diameter and having a hole three-quarters of an inch in diameter in the center. The cap is held to the shell by means of four small brass screws. The receiver is now completed and should give a loud click each time that a battery is connected or disconnected from the two posts, _A_ and _B_. The original Bell telephone apparatus was made up simply of two receivers without any battery or transmitter. In such a case the current is generated by "induction." The receiver is used to speak through as well as to hear through. This method of telephoning is unsatisfactory over any appreciable distances. The time utilized in making a transmitter will be well spent. A simple form of transmitter is shown in Figure 148. The wooden back, _B_, is three and one-half inches square and three-quarters of an inch thick. The front face of the block is hollowed out in the center as shown in the cross-section view. The face-plate, _A_, is two and one-half inches square and one-half an inch thick. A hole, seven-eighths of an inch in diameter, is bored through the center. One side is then hollowed out to a diameter of one and three-quarter inches, so as to give space for the diaphragm to vibrate as shown in the cross-sectional drawing. The carbon buttons are one inch in diameter and three-sixteenths of an inch thick. A small hole is bored in the center of each to receive a brass machine screw. The hole is countersunk, so as to bring the head of the screw down as close to the surface of the carbon as is possible. Then, using a sharp knife or a three-cornered file, score the surface of the carbon until it is covered with criss-cross lines.