pressure upon D produce variations of intensity in the current. This crude instrument is so delicate that even the tread of a fly produces a sufficient change of pressure, and consequent change of intensity in
Fig. 13.
the current, to be distinctly heard in the receiving telephone. Fig. 14 shows another form of transmitter. Two pieces of gas-carbon, C C, are stuck to a pine board with sealing-wax, and connected with the wires of a battery. A third piece of carbon, A, pointed at the ends, rests loosely between them. This instrument will transmit low sounds
Fig. 14.
uttered at a distance from it of several yards. The capacity of the two instruments last described for transmitting sounds seems to depend upon the fact that the current is made to pass through an imperfect contact, which, when acted upon by the vibrations of sound, gives to the current an undulatory character. Successful experiments have been made with loose-jointed machinery used as a transmitter. Even a common nail laid loosely across two other nails, insulated from each other but connected with a battery, will make a good transmitter.
