1026 Dynamic Theory. 



marked c in the figure which run back to the bind screws d, where they 

 connect with the main circuit No battery is required in this instru- 

 ment, the magnet of the transmitting instrument inducing the needful 

 current to act upon the magnet of the receiver. According to S. P. 

 Thompson the theory of this instrument is as follows: " The magnet a 



FIG. 400. 



Figure 400. Section of a Telephone, a. Steel magnet. 6. Bobbin of wire on the 

 end of a. c. Wire from the bobbin to d. d. Binding screws forming connection with 

 the general circuit, e The vibrating diaphragm. /The external wooden or rubber 

 case, fir. The mouth piece with hole at the bottom opening on the diaphragm. 



induces a certain number of lines-of-force through the coil b. Many 

 of these pass into the iron disc. When the iron disc in vibrating moves 

 toward the magnet pole, more lines of force meet it; when it recedes 

 fewer lines of force meet it. Its motion to and fro will therefore alter 

 the number of lines-of-force which pass through the hollow of the coil 6, 

 and will therefore generate in the wire of the coil, currents whose 

 strength is proportional to the rate of change in the number of the 

 lines of force which pass through the coil." (See ch. 36 for explana- 

 tion of ' < lines-of-force. ") 



Dr. Konig showed that if the diaphragm be removed, and a tuning 

 fork set in vibration near the end of the magnet, the disturbance of the 

 lines-of-force takes place the same as with the vibratory disc, and a fork 

 of the same pitch, or differing by octaves, when placed near the magnet 

 of the receiving instrument from which the disc was also removed, 

 took up the vibration and gave its fundamental sound. Moreover if 

 the circuit be broken so that the vibration of the fork does not set up 

 currents it will continue to vibrate a much longer time than when the 

 circuit is closed and it is encountering resistance and doing work. In 

 short we see here the same principle as that involved in the action of 

 the dynamo. The telephone is in fact a sort of dynamo. But it is a 

 dynamo of such delicacy that its currents are generated and fluctuated 

 by exceedingly small disturbances of the field of its magnetism. 



It is commonly explained that this disturbance is due to and exactly cor- 

 responds with the vibrations of the disc or diaphragm of the transmit- 

 ting instrument. But now the question arises, is this vibration of the 

 heavy body the real final cause of the disturbance of the magnetic field, or 

 is it not rather caused by the accompan} r ing movement of the condensed 



