336 



BELL SYSTEM TECHXICAL JOURNAL 



seen that llic maxiimim iiiiinbcr of riirrent \aliies which can he 

 employed is determined 1)\' the maximum powir which it is per- 

 missible to use. 



In the case of land line tcletirai)h circuits operated with cUrect 

 currents, it is well known that cjuadruplex circuits are much more 

 seriously affected by fluctuations and interference than are circuits 

 employing only two current values. (A quadruplex telegraph circuit 

 employs four current values for transmission in one direction.) In 

 general, it may' be said that the possibilities of improxing ordinary 

 direct-current operated telegraph circuits in this manner do not 

 appear particularly promising. 



In the case of wireless transmission over great distances all three 

 of the above factors are important in limiting the number of current 

 values which can be effecti\ely employed. In the first place, as is well 

 known, large variations take place in the efficiency of the transmitting 

 medium so that the received signals vary considerably in magnitude 

 from time to time. Secondly, the interference, at least at certain 

 seasons, is great enough to make it difficult to distinguish between 

 the current values e\cn when the usual method which employs only 

 two current xalues is employed. Thirdly, the recei\ed power is 

 limited because of the great attenuation suflered by the wireless 

 waves. 



In the case of carrier transmission, it may be tliat there will be a 

 field for the use of more than two current values. The relative 

 cheapness of the line circuits, however, will tend to limit the amount 

 by which it will be economical to increase the cost and complexity 

 of the recei\ing apjiaratus. Moreover, it should be borne in mind 

 lliat no allowance has been made for the effect on the line speed of 

 increasing the number of current xalues, this being considered outside 

 the scope of the jiresent paper. 



Changing an existing network of ti'legrapii tircuils so as to employ 

 a code with three instead of two current xalues would require new 

 types of telegraph repeaters as well as new sending and receixing 

 apparatus, and new operating methods. It is considered to be out- 

 side of the scope of this pajier to go into a discussion of the details of 

 this matter. 



".Si NIC W.wi-:" SvsTKMs 



Considerable interest and discussion has been created by sug- 

 gestions which have been made to use so-called "sine wave" systems 

 of telegrajihy. In xiexx- of this, a brief discussion of these systems 

 is given below. 



