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THE POPULAR SCIENCE MONTHLY 



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 



Trn 



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 



Of) o oj^ 



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. 



