TRANSMISSION OF INFORMATION 537 



sending operator has at his disposal three positions of a sending key 

 which correspond to applied voltages of the two polarities and to no 

 applied voltage. In making a selection he decides to direct attention 

 to one of the three voltage conditions or symbols by throwing the key to 

 the position corresponding to that symbol. The disturbance trans- 

 mitted over the cable is then the result of a series of conscious selec- 

 tions. However, a similar sequence of arbitrarily chosen symbols 

 might have been sent by an automatic mechanism which controlled 

 the position of the key in accordance with the results of a series of 

 chance operations such as a ball rolling into one of three pockets. 



Fig. 1 



Owing to the distortion of the cable the results of the various 

 selections as exhibited to the receiver by the recorder trace are not 

 as clearly distinguishable as they were in the positions of the sending 

 key. Fig. 1 shows at A the sequence of key positions, and at B, 

 C and D the traces made by the recorder when receiving over an 

 artificial cable of progressively increasing length. For the shortest 

 cable B the reconstruction of the original sequence is a simple matter. 

 For the intermediate length C, however, more care is needed to dis- 

 tinguish just which key position a particular part of the record repre- 

 sents. In D the symbols have become hopelessly indistinguishable. 

 The capacity of a system to transmit a particular sequence of symbols 

 depends upon the possibility of distinguishing at the receiving end 

 between the results of the various selections made at the sending end. 

 The operation of recognizing from the received record the sequence 

 of symbols selected at the sending end may be carried out by those 

 of us who are not familiar with the Morse code. We would do this 

 equally well for a sequence representing a consciously chosen message 

 and for one sent out by the automatic selecting device already referred 



