LOADED TELEGRAPH CABLES 



395 



centage of the received voltage and are practically negligible in their 

 effect upon the thermocouple. 



Measurement of the Time of Propagation 



The time of propagation of a steady state sinusoidal voltage over a 

 loaded cable of transatlantic length is of the order of 0.3 second. 

 It is measured by means of the circuit shown in Fig. 2, which is 



LOCAL 

 EARTH 



SEA EARTH 



Fig. 2. 



operated simultaneously at both ends of the cable. At each end a 

 perforated tape is prepared which when inserted in the high speed 

 transmitter 7" will cause a train of about ten reversals to be sent out 

 over the cable. The potentiometer P is adjusted so that a measurable 

 record of either transmitted or arriving trains, depending upon the 

 position of the key k, will be obtained on the string oscillograph O 

 after amplification by the vacuum tube amplifier A. The condenser 

 C is inserted between the cable and transmitter in order to remove the 

 low frequency components of the transient part of the train, which 

 would otherwise overwhelm the steady state component at the distant 

 end of the cable. The oscillograph, shown in Fig. 3, gives a continuous 

 record of the current in a fine wire, which is free to respond to the 

 interaction between the current and the strong magnetic field in which 

 the wire is placed. The displacement of the wire, and hence the 

 amplitude of current in it, is recorded on a long strip of sensitized 

 paper, which is developed and fixed within the camera by a continuous 

 process immediately after exposure. By this means it is possible to 

 obtain a continuous record, over a period of several minutes, of 

 voltages transmitted and received over the cable. A second wire can 

 be used to give simultaneously a record of any other current which 



