398 BELL SYSTEM TECHNICAL JOURNAL 



a final check, records similar to Fig. 4 are taken with various times 

 elapsing between reception and transmission at the second station. 

 If an error exists in the timing arrangement its effect on the time of 

 propagation will be greater the greater the interval between receiving 

 and sending, and the time of propagation corresponding to negligible 

 error in the timing system can be easily obtained by graphical methods. 

 The error of measurement of the time of propagation is probably less 

 than 1 per cent. 



The inductance of a loaded conductor is an inci"easing function of 

 current for the range of current values used in cable practice be- 

 cause of the increase of permeability of the permalloy, and since 

 with finite transmitting voltage the current at the sending end 

 may be quite large, the inductance of this portion of the cable 

 under such conditions will be larger than the value it would have 

 for very small current in the conductor. Accordingly the time of 

 propagation at a given frequency will be a function of voltage. 

 The value of inductance corresponding to very small current in the 

 conductor can be derived from the time of propagation corresponding 

 to zero transmitted voltage, which is obtained by extrapolation from 

 measurements of the time of propagation at several values of trans- 

 mitted voltage. 



Measurement of Capacity 



The dielectric capacity of submarine cables in the telegraph range 

 of frequencies is in general comparatively insensitive to changes in 

 temperature and hydrostatic pressure, so that it is possible to estimate 

 this quantity rather accurately at various frequencies by means of 

 measurements made in the factory, the factors required to reduce 

 the results of the measurements to sea bottom conditions being 

 relatively easy of determination. In order to check these values, 

 however, the d.c. capacity of the laid cable is measured by the method 

 of mixtures, employing a charging time of lo seconds or more and a 

 mixing time of equal duration. 



Computation of Cable Parameters 

 The inductance of the cable can be computed at any frequency 

 from the measured values of capacity and time of propagation by 

 means of equation (8), proper allowance being made for the rather 

 small effect of resistance. 



Having computed the inductance and the capacity of the cable, 

 only the resistance and the leakance remain undetermined. The 

 direct current resistance can be computed from factory measurements 

 and checked by measurement on the cable. The resistance component 



