LOADED TELEGRAPH CABLES 399 



due to eddy currents in the loading material can be computed from 

 the resistance measurements obtained in the factory in the process of 

 determining the inductance of sample core lengths. The eddy current 

 resistance is proportional to the square of the product of frequency and 

 permeability, and corresponding reduction factors must be employed 

 in computing the eddy current resistance of the laid cable from the 

 factory measurements. Since we are dealing with values of the 

 parameters corresponding to very small current in the cable conductor, 

 the hysteresis resistance is zero. In addition to the losses in the 

 loading material there are other losses peculiar to continuously loaded 

 cables due to currents Induced in the cable structure. The loading 

 material is ordinarily applied to the conductor in the form of a tape 

 or wire of finite width, so that it has a definite lay, and since the 

 magnetic flux in the loading material tends to follow the convolutions 

 of the latter there is a component of this flux parallel to the axis of 

 the central conductor. Consequently as the flux changes with signal 

 current, electromotive forces are induced in those portions of the 

 cable structure which link with it— the teredo tape and armor wires, 

 for example. The resulting energy loss has in most practical cases 

 comparatively small effect on the performance of the cable, and the 

 magnitude of the corresponding resistance component can be estimated 

 by theoretical methods and by measurements in the factory. The 

 various components of resistance having been estimated, the total 

 resistance at any frequency can be computed. Likewise the value of 

 dielectric leakance of the laid cable at any frequency can be estimated 

 from tests made during manufacture. These values of resistance and 

 dielectric leakance should be considered merely as first approximations, 

 since they are based in part on assumptions that cannot be directly 

 verified. 



Formula (2) is then employed to determine the effect upon the 

 attenuation constant of departures from the approximate values of 

 resistance and leakance, and by comparing these results with the 

 measured values of attenuation constant, mutually consistent sets of 

 values of resistance and of dielectric leakance can be computed at 

 various frequencies. A choice of the best sets of values can then be 

 made, due weight being given to the evidence available from computa- 

 tions and laboratory measurements regarding the manner in which 

 these quantities vary with frequency. 



From the curves relating the values of measured attenuation 

 constant and the transmitted voltage, a check can be made of the 

 method of computing the increase in attenuation due to hysteresis and 

 to variation of inductance with current.'' Since this method employs 



«See Buckley, loc. cit., and U. Meyer, E. N. T., Vol. 3, No. 1, 1926. 



