300 TRANSATLANTIC TELEPHONING. 



thus to open and close a new circuit, it is called a repeater, and on a 

 land line such repeatei's ma}'^ be introduced as often as ma}^ be neces- 

 sary to transmit the message as far as we please. 



On an ocean cable, however, it is impossible to introduce repeaters, 

 and the only thing to be done is to construct receiving instruments of 

 extreme delicacy, capable of responding to the greatly enfeebled elec- 

 tric impulses. But the impulses on an ocean cable are not merely 

 enfeebled. There is another difficulty more serious still. In conse- 

 quence of what is called the capacity of the cable, the impulses are 

 spread out or prolonged, so that a momentary impulse started at the 

 sending end reaches the receiving end much prolonged. It may help 

 to an understanding of what takes place if we consider a case more in 

 line with every day experience. Suppose we try to transmit messages 

 by sending pull's of air into a long tube. It is evident that we should 

 succeed better if the tube be narrow than if it be widened into a 

 chamber of considerable capacity where the puffs sent into the tube 

 would make little impression, and where they would find room to 

 spread out and become not only enfeebled, but prolonged. An ocean 

 cable is just such a chamber or reservoir for electric impulses. It has 

 a large capacity for an electric charge. Such impulses as we use on 

 land lines make little impression upon it, and such effects as are pro- 

 duced at the receiving end are so prolonged that they lose all their 

 character as dots and dashes. It is possible, however, to adopt our 

 sending to this condition. We can wait. We can allow a sufficient 

 interval between the successive impulses to give time for each to pro- 

 duce its effect at the receiving end. On an ocean cable we can tele- 

 graph, but we must telegraph slowly. 



Very different is the problem of transmitting speech. Everyone 

 knows that audible sound is the result of vibrations in the air. The 

 differences that we recognize between sounds must be due to differences 

 between these vibrations or sound waves. To each sound must corre- 

 spond its own sound wave, distincth^ different from all the others. 

 It is wonderful, even when the air alone is the medium, that these dis- 

 tinctive differences should be preserved and that we should be able to 

 recognize such a great variety of sounds. It is still more wonderful 

 when we study the sound waves and find in what small differences the 

 distinction between different sounds consists. Far more wonderful 

 still is it when we consider all that must take place in the several trans- 

 formations between the speaker and the hearer when sound is trans- 

 mitted by telephone. 



We speak against a thin sheet-iron disk a little larger than a dollar. 

 The vibration is communicated to the disk, and this, through a deli- 

 cately adjusted mechanism, gives rise to electric waves which traverse 

 the wire and, in the receiving instrument, produce vibrations in another 

 disk, which communicates them to the air and so to the ear. Through 



