DEVELOPMENT OF TELEPHONE SERVICE 409 



Unlike the hand telephone in every respect, the Blake transmitter 

 consisted of a small black-walnut box, nearly square in form and 

 having a funnel-shaped hole cut in the door to serve as a mouth- 

 piece. Within the box was a soft iron diaphragm and suspended par- 

 allel to its center was a polished button of pure carbon; between the 

 two hung a German-silver spring bearing a pellet of platinum which 

 barely touched the center of the carbon. When the Blake transmitter 

 was in use, the impinging sound waves pressed the diaphragm against 

 the platinum and forced it with varying pressure against the carbon 

 button. This changing pressure varied the resistance offered to the 

 flow of the battery current, which pulsated through the carbon and 

 into the primary winding of an induction coil or transformer, where 

 it was converted into an alternating current through the inductive 

 effects of the secondary winding and passed out in undulating or wave- 

 like form into the line or subscriber circuit, thence through the 

 copper wire in the green-covered telephone cord attached to the re- 

 ceiver, and on into the wire wound on the electro-magnets. Ener- 

 gizing the latter varied the attractive or pulling power of the pole 

 pieces, thus causing the receiver diaphragm to vibrate in a manner 

 exactly reproducing the vibratory motion of the transmitting dia- 

 phragm and setting up a series of sound waves in the receiver exactly 

 corresponding to those produced by the vocal cords of the speaker. 



So sensitive is a properly adjusted telephone diaphragm that its 

 vibrations may cause several hundred thousand variations a minute 

 in pressure of platinum point on carbon button in the Blake trans- 

 mitter, or between carbon granules in certain other microphonic forms 

 Xaturally the amount of current thus passing through this carbon 

 gateway is extremely small, depending principally on the pitch of 

 the speaker's tone and the physical condition of the line. Under 

 ordinary circumstances and with both telephones and a complete cop- 

 per circuit in good condition, distinct transmission of speech only 

 requires a maximum generation of about one tenth of a milliampere 

 of current at any one period, or only a millionth part of the current 

 required to light an incandescent lamp. Again, probably only one 

 fourth or less of even this infinitely small amount of current reaches 

 the electro-magnets in the receiver, the other portion being used up 

 in overcoming resistances. Where the circuit is three or four hundred 

 miles in length, it is probable ' that only about one one-hundredth 

 of the original current produced at the transmitting station is finally 

 utilized at the receiving station.' 



Where operating companies desired a less expensive instrument 

 than the standard Blake set, for use of small users of service, only 

 willing to pay a low rate, a much cheaper set (Fig. 33) was supplied. 

 This set was originally intended to be used only on private lines, or 



