Transmitters of the type just described work well over short distances, but the currents they produce are too feeble for transmission over a very long wire, and on this account they have been superseded by transmitters on the microphone principle. A microphone is an instrument for making extremely small sounds plainly audible. If a current is passed through a box containing loose bits of broken carbon, it meets with great resistance, but if the bits of carbon are compressed their conducting power is considerably increased. Even such slight differences in pressure as are produced by vibrating the box will affect the amount of current passing through the carbon. If this current is led by wires to an ordinary telephone receiver the arrangement becomes a simple form of microphone. The vibrations of the box vary the resistance of the carbon, and the corresponding variations in the current set up vibrations in the receiver, but in a magnified form. The smallest sound vibrations alter the resistance of the carbon, and as these vibrations are magnified in the receiver, the reproduced sound is magnified also. The footsteps of a fly may be heard quite distinctly by means of a good microphone, and the ticks of a watch sound like the strokes of a hammer. [Illustration: FIG. 31.--Diagram of Microphone Transmitter.] By means of this power of magnifying vibrations a microphone transmitter can be used on a line of tremendous length, where an ordinary Bell transmitter would be utterly useless. The general features of this transmitter, Fig. 31, are a diaphragm and a block of carbon separated slightly from one another, the intervening space being filled with granules of carbon. These are enclosed in a case of ebonite having a mouthpiece in front and two terminals behind, one terminal being connected with the carbon block and the other with the diaphragm. From these terminals wires are led to a battery and to the receiver, which is of the Bell type. The current has to pass through the carbon granules, and the movements of the diaphragm when set in vibration by the voice vary the pressure upon the granules, and in this way set up variations in the current. Carbon dust also may be used instead of granular carbon, and then the instrument is called a “dust transmitter.” [Illustration: FIG. 32.--Combined Telephone Transmitter and Receiver.] It is usual to have a transmitter and a receiver on one handle for the greater convenience of the user. The arrangement is shown in Fig. 32, and it will be seen that when the user places the receiver to his ear the transmitting mouthpiece is in position for speaking. The microphone with its carbon dust is placed at A, just below the mouthpiece, and the earpiece or receiver B contains a little magnet and coil with a diaphragm in front, so that it is really a Bell instrument. A little lever will be noticed at C. This is a switch which brings the transmitter into circuit on being pressed with the finger.