INDUCTIVE LOADING FOR TELEPHONE FACILITIES 163 



the early stages of cable manufacture. This was done by refinements in the 

 cable drying treatments. 



(3) Changing Fields of Use for Iron-Wire Core Loading Coils 



The new coarse-gauge cable loading coils, above referred to, marked the 

 beginning of the use of 65-permeability iron-wire in place of 95-perme- 

 ability wire in the cores of standard cable loading coils. 



In every respect except permeability, the 65-permeability wire was su- 

 perior to the higher permeability wire. The lower permeability was relatively 

 disadvantageous as regards d-c resistance per unit inductance in coils of a 

 given size. On the other hand, the core-loss resistance was substantially 

 sihaller, by virtue of the lower permeability and the superior hysteresis 

 characteristics. In consequence, the total effective resistance of the 65- 

 permeability core coils was lower at the upper speech-frequencies and nearly 

 the same at the important middle frequencies, so that there was considerably 

 less attenuation-frequency distortion. 



Other even more important service advantages of the 65-permeability 

 core toll cable loading coils resulted from their much greater magnetic 

 stability. D-c signaling currents caused smaller temporary changes in in- 

 ductance and effective resistance, in consequence of the superimposed d-c 

 magnetization. Also, the residual effects of strong superimposed currents, 

 manifested as permanent or semipermanent changes in inductance and 

 effective resistance, were much smaller. 



A specially valuable advantage of the 65-permeability wire core-material 

 was in the substantially smaller amount of telephone transmission distortion 

 caused by the operation of superposed composite telegraph systems. The 

 transient core-magnetization caused by the telegraph currents caused small 

 transient changes in the inductances of the coils, and relatively very large 

 transient changes in the effective resistances. The resulting non-linear 

 distortion became known as "telegraph flutter." It varied as a function of 

 telephone frequency and telegraph speed, the size of the core, the inductance 

 of the windings, and the ratio of the amplitudes of the telephone and tele- 

 graph currents. It was accumulative in effect as the circuit lengths increased. 

 Since simultaneous telephony and telegraphy was very general and was 

 important from the revenue standpoint in the open-wire and cable long- 

 distance facilities, the control of "telegraph flutter" became an increasingly 

 important requirement in the development of new loading coils. 



(The need for satisfactory control of "telegraph flutter" eventually led 

 to the development of the improved cable telegraph systems which are 

 described in Section 8 of this review.) 



By 1912, the use of 95-permeability core-material in new toll cable coils 



