means had not been developed for using the wires simultaneously 

 for both telephone and telegraph, and the two services were offered 

 alternatively to the private line customers. Beginning in 1887, how- 

 ever, successful experiments were conducted in using telephone wires 

 simultaneously for telephone and telegraph services by the method of 

 superposition which is shown in Fig. 6. 



The first method, called "simplexing," is an adaptation of the 

 phantom principle for the use of telegraph on telephone circuits, the 

 grounded telegraph circuit being introduced at the midpoint of re- 

 peating coils at the two ends of the telephone circuits, the currents 

 dividing equally in opposite directions so that there is no interference 

 between the telephone and telegraph circuits. The other method of 

 simultaneous operation of telephone and telegraph circuits, however, 



TO 



TELEPHONE 

 APPARATUS 



TELEGRAPH 

 APPARATUS 



TO 



TELEPHONE 

 APPARATUS 



TELEGRAPH 

 APPARATUS 



■X" 



Fig. 6 — Schematic of telegraph circuit superposed on a telephone circuit using the 



"simplex" method. 



depends upon a new principle and one which has come to be of the 

 greatest importance in the subsequent development of telephony. 

 This principle is the selection and separation of electric currents into 

 different channels depending upon differences in their frequency of 

 alternation. 



While the form of the electrical oscillations which transmit speech 

 over a telephone circuit is extremely complicated, as indicated in Fig. 

 1, such oscillations can, by processes of analysis, be considered as made 

 up of a large number of simple alternating currents of different fre- 

 quencies. A simple current of this type, which is sometimes spoken 

 of as a sine wave because of the mathematical law which expresses the 

 variation in the flow with time, is shown in Fig. 7. Such a current by 

 gradual variations at regular intervals reverses its direction of flow. 

 Each double reversal is called a "cycle" and the number of such double 

 reversals in a second is called the frequency of cycles per second. 



11 



