Figure 140 shows how to arrange a regular telegraph line for two stations. Gravity batteries should be used for regular telegraph work. It is necessary that the key should be kept closed by having its circuit-closer shut when messages are not being sent. If one of the keys is left open the circuit is broken, and it is not possible for a person at the other end of the line to send a message. Every telegraph office has a name or call usually consisting of two letters; thus for New York the call might be N. Y. and for Chicago, C. H. If New York should desire to call Chicago, he would repeat the call letters, C H., until answered. Chicago would answer by sending I, several times and signing, C H. When so answered, New York would proceed with the message. CHAPTER XI MICROPHONES AND TELEPHONES In 1878, David Edward Hughes discovered that the imperfect contact formed between two pieces of some such substance as carbon or charcoal is very sensitive to the slightest changes in pressure, and when included in an electric circuit with a battery and a telephone receiver, will transmit sounds. Such an instrument is called a _microphone_. It has various forms but in most of them one piece of carbon or charcoal is held loosely between two other pieces in such a manner as to be easily affected by the slightest vibrations conveyed to it through the air or any other medium. [Illustration: Fig. 143.—A Microphone connected to a Telephone Receiver, and a Battery.] Figure 143 illustrates a simple form of instrument embodying this principle. A small pencil of carbon is supported loosely between two blocks of the same substance glued to a thin wooden sounding-board of pine. The sounding-board is mounted in an upright position on a wooden base. The carbon pencil rests loosely in two small indentations in the carbon blocks. The blocks are connected, by means of a very fine wire or a strip of tinfoil, with one or two cells of battery and a telephone receiver. Any vibration or sounds in range of the microphone will cause the sounding-board to vibrate. This will affect the pressure of the contact between the carbon pencil and the two blocks. When the pressure between the two is increased the resistance in the path of the electric current is decreased and more current immediately flows through the circuit. On the other hand, when the pressure is decreased, the resistance is increased and less current flows through the telephone receiver. The amount of current flowing in the circuit thus keeps step with the changes in the resistance, and accordingly produces sounds in the telephone receiver. The vibrations emitted from the receiver are usually much greater than those of the original sounds, and so the microphone may be used to magnify weak sounds such as the ticking of clock-wheels or the footfalls of insects. If a watch is laid on the base of the microphone, the ticking of the escapement wheel can be heard with startling loudness.