ELECTRICAL INVENTIONS 219 



ran to the ground, the other was joined to the wire wound about 

 the transmitter core. From this coil the current ran through a 

 wire connecting with the other station, where, after passing 

 through the coil about the receiver core, it was carried to the 

 ground which served to complete the circuit. When one spoke 

 into the transmitter, his voice caused a vibration of the dia- 

 phragm. As the diaphragm bent toward or away from the soft 

 core, magnetized by the flowing current, it caused a fluctuation in 

 the intensity of the current, because it was itself an induced 

 magnet moving in relation to a wire coil. These changes in the 

 intensity of the current 

 carried from the trans- 

 mitter caused the core of 

 the receiver to vary the 

 magnetic pull on its dia- 

 phragm in accordance with 

 the vibrations of the dia- 

 phragm of the transmitter, 

 and the receiving dia- 

 phragm vibrated so as to re- 

 produce the speaking voice. 

 In 1856 Du Moncel 

 discovered that, when a 

 rod Of carbon forms part FIG. 8 9 .- Diagram of a microphone transmitter 



of an electric circuit, compression of the carbon facilitates 

 the flow of the circuit. In 1877 Edison applied this principle 

 for making an improved telephone transmitter (Fig. 89). The 

 diaphragm of this transmitter rests lightly against carbon gran- 

 ules held in a shallow cup of hard rubber. As the current 

 introduced through metal strips flows through these carbon 

 particles, its intensity is increased or decreased according to the 

 pressure of the diaphragm. This makes a much more sensi- 

 tive transmitter than the earlier type. 



In its early history, when two people wished to talk to each 

 other over the telephone, their two instruments were connected 



